The First Topic: The Cause (Reason, 'Illah) of Riba in Riba-Related Commodities.
The First Issue: The Cause of Riba in the Four Riba-Related Commodities
The Muslim scholars (Ulama') have differed regarding the cause of Riba (usury) in the four Riba-related commodities—wheat, barley, dates, and salt—leading to various opinions. The most prominent of these opinions are two:
The First Opinion: The cause or reason ('illah) of Riba (Usury) in the four Riba-based commodities—wheat, barley, dates, and salt—is edibility (ta'um) combined with measurability by volume (kayl) or weight (wazn). This is the old opinion of Al-Shafi'i [45] - See: "Al-Majmu'" by Al-Nawawi(9/397) , a narration from Ahmad [46] -"Al-Mughni" by Ibn Qudamah (4/6). Refer to: ""Al-Fatawa al-Kubra"   "by Ibn Taymiyyah (5/391). , the view of Sa'id ibn al-Musayyib [47] - It was narrated from Sa'id ibn al-Musayyib. See: "Al-Mughni "by Ibn Qudamah (4/6). , and the one chosen by Ibn Taymiyyah [48] - Ibn Taymiyyah said: "The cause ('illah,(علة  for the prohibition of Riba al-fadl is the combination of edibility(ta'um)  with measurability by volume( kayl الكيل،) or weight (waznالوزن، ). This is a narration from Ahmad." "Al-Fatawa al-Kubra"(5/391). He also said: "There has been a historical and contemporary disagreement over many issues related to Riba. They have differed on the prohibition of disparity tafadul in the six commodities: gold, silver, wheat, barley, dates, and salt. The question is whether the basis is equivalence (tamathul)—that is, measurability by volume or weight—or whether it is value (thamaniyyah) and edibility, or whether it is value and equivalence along with edibility, nourishment, and what sustains it. Or is the prohibition without reason (ghayr mu'allal), confined solely to what the textual evidence indicates? These are well-known opinions. The first is the opinion of Abu Hanifah and the most famous narration from Ahmad. The second is the view of Al-Shafi'i and Ahmad in another narration. The third is another narration from Ahmad, preferred by Abu Muhammad, and it is close to the view of Malik, which is stronger than the others. The fourth is the view of Dawud and his companions, and it is also reported from Qatadah." ""Majmu' al-Fatawa"", (29/470). , Ibn 'Uthaymeen [49] -Ibn ‘Uthaymeen said: “As for the cause('illah) in the four commodities, it is that they are measurable by volume (makil) and edible (mat'oom), meaning that the cause is composed of two factors: measurability by volume( kayl)   and edibility (tu'um). This is the reality: they are both measured by volume and edible. The impact of the difference of opinion is apparent in certain examples: if someone sells a sa'ah a measure of ushnan a type of cleaning material for two sa'ahs of the same, then if we say the cause is measurability by volume (kayl), it is not permissible. If we  say the cause is edibility (tu'um), it is permissible. If we say the cause is both measurability by volume and edibility, it is also permissible. If one sells a fruit for a larger quantity of the same type, then if we say the cause is edibility, it is not permissible. If we say the cause is measurability by volume, it is permissible. If we say the cause is both measurability by volume and edibility, it is permissible. These examples are based on the differences regarding the determination of the cause. If someone says: 'We agree that wheat, barley, and dates are edible, but what about salt?' Shaykh al-Islam Ibn Taymiyyah responded that salt makes food better, and is therefore associated with it. Hence, it is said: 'Grammar in language is like salt in food,' because salt is an essential component of food. Based on this reasoning, Riba also applies to spices that enhance food, as they are also associated with it. When we reflect on these three opinions, we find: Firstly, the closest to the correct view is this one; the reasoning being that when we contemplate the six commodities specified by the Messenger peace and blessings be upon him, we find that they are edible and measurable by volume. Secondly, the fundamental principle in buying and selling is permissibility hill, so we cannot prohibit people from engaging in what is fundamentally permissible unless it becomes clear to us in an explicit manner. Since the only clear cases are those in which both measurability by volume and edibility are present, we say: anything else remains upon the original ruling of permissibility.” "Al-Sharh al-Mumti' "(8/397).He also said: “This contains an indication of the cause (‘illah) of Riba, which is edibility tu'um, but there is no doubt—on this interpretation—that it does not refer to every edible item; otherwise, even water would be included, as water, when unrestricted, is considered part of food (ta'am)."Allah said in the Qur'an: " So whoever drinks thereof, he is not of me, and whoever tastes it not, he is of me, except him who takes thereof in the hollow of his hand"(Al-Baqarah: 249). However, what is meant by (ta'am) food is that which is eaten as a staple provision qut, for the people during the time of the Messenger peace and blessings be upon him would eat barley as a staple food, meaning it was their primary sustenance. Based on this, there is an indication in this hadith to the stronger opinion in this matter, which is that the cause ('illah) of Riba in the four categories mentioned in the Hadith of ‘Ubadah ibn al-Samit is edibility (tu'um), but we also add to that measurability by volume ( kayl), as all the narrations related to this issue indicate measurement by volume. Based on this, food that is both edible and measured by volume, and which is used as a staple provision by people, is subject to Riba. As for food that is not measured by volume, or anything that is not quantified by volume, Riba does not apply to it, such as fruits of various kinds, vegetables, sidr lote tree fruits, ushnan a type of plant, henna, and similar items." "Fath thil-Jalal wal-Ikram"( 4/31). ,
and the "Fatawa al-Lajnah al-Da’imah" [50] - The fatwa explicitly mentions measurability by volume( kaylكيل )   along with edibility (tu’um). It is stated in the fatwa of the Permanent Committee (the Fatawa al-Lajnah al-Da’imah ): “The items in which Riba is prohibited are: gold, silver, wheat, barley, dates, and salt, as well as anything that shares the cause (‘illah) of Riba with these six categories. The cause in the two currencies gold and silver is their value (thamaniyyah), and in the rest of the categories, it is measurability by volume (kayl)   along with edibility(tu’miyyah), according to the correct view among the scholars.” "Fatawa al-Lajnah al-Da’imah "– First Group (Collection), (13/268). .
That is for the following reasons:
Firstly: Each of these attributes has its impact, and the ruling is contingent upon all of them in the stated text. Therefore, it is not permissible to omit any of them [51] - Refer to: "Al-Mughni "by Ibn Qudamah (4/6). .
Secondly: The measure ((الكيل, weight(الوزن), and kind(Type) do not necessarily entail the requirement of equivalence. Rather, their effect lies in fulfilling the rationale that necessitates the establishment of the ruling, not the condition that ensures its realization. Edibility alone does not establish equivalence due to the absence of a Sharia-based criterion for it [52] - - Refer to: "Al-Mughni "by Ibn Qudamah (4/6). .
Thirdly: The hadiths that address this issue must be reconciled with one another, and each one must be understood in light of the other. Thus, the Prophet Muhammad's صلى الله عليه وسلم prohibition of selling food except in like-for-like measures is restricted to that which has a Sharia-based criterion, namely volume or weight. Likewise, his prohibition of selling one Saa'((صاع (a unit of measurement) in exchange for two Saa's  (صاعين)is restricted to consumables in which disparity is forbidden [53] - Refer to the above reference .
The Second Opinion: The underlying cause(ʿillah, علة) of Riba (usury) in the four categories, in the context of Riba al-fadl (Usury of Excess = Excess-based usury), is their suitability for storage and sustenance [54] - The meaning of iqtīt (suitability for sustenance) is that the food is muqtāt (used as sustenance), meaning it is something upon which the human body can subsist when relying solely on it. The meaning of iddikhār (storability) is that the food does not spoil when delayed, except if the delay exceeds what is customary. Refer to: "Mawāhib al-Jalīl" by al-Hattāb (6/198). , while in the context of Riba al-nasi'ah (Interest of Deferred Payment), it is edibility [55] - The mere characteristic of being taʿām (edible), whether the food is storable and suitable for sustenance (muqtāt) or not, provided that it is not used for medicinal purposes, does not fall under the category of Riba al-nasīʾah (Interest of Deferred Payment). Therefore, according to their view, it is permissible to have disparity (tafāḍul) in non-storable consumables, but deferment (nasāʾ) is not permissible for them. . This is the view of the Maliki [56] Refer to:"Al-Sharḥ al-Kabīr" by al-Dardīr along with the commentary "Ḥāshiya) of al-Dusūqī" (3/47), and "Minah al-Jalīl" by ʿUlaysh( 5/5). school and was chosen by Ibn al-Qayyim [57] - Ibn al-Qayyim said: "The Lawgiver explicitly prohibited Riba al-fadl (usury of excess) in six commodities: gold, silver, wheat, barley, dates, and salt. There is consensus among the scholars on the prohibition of disparity (tafāḍul) when these items are exchanged with the same kind. However, they differed regarding items beyond these six. Some scholars restricted the prohibition to these specific items, with the earliest known proponent of this view being Qatādah. This is also the opinion of the Ahl al-Ẓāhir (literalists) and was chosen by Ibn ʿAqīl in his later writings, despite his general acceptance of qiyās analogical reasoning. He said, "The rationales used by those who rely on qiyās in the issue of Riba are weak, and when the underlying cause is unclear, analogy is invalid." Another group prohibited Riba in everything that is measured by volume or weight when exchanged within the same kind. This is the view of ʿAmmār, the apparent position of Ahmad, and the opinion of Imam Abu Hanifah. Another group restricted the prohibition to foodstuffs, even if they are neither measured nor weighed. This is the opinion of Imam al-Shāfiʿī and one narration from Imam Ahmad. Another group restricted it to foodstuffs that are either measured or weighed, which is the view of Saʿīd ibn al-Musayyib, one narration from Ahmad, and one opinion of al-Shāfiʿī. Another group restricted it to items that are considered qūt (staple food) and those that complement it. This is the opinion of Imam Mālik and is the most correct of these views."  "ʿIʿlām al-Muwaqqiʿīn"(2/174).   .
The Evidences:
Firstly:  From the Sunnah
 It is narrated from ʿUbādah ibn al-Ṣāmit,  رضي الله عنه, that the Messenger of Allah صلى الله عليه وسلم said: “Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt—like for like, equal for equal, hand to hand. But if these categories differ, then sell as you wish, provided it is hand to hand.” [58] - Reported by Muslim 1587.
Reasoning:
The Hadith indicates that the underlying cause (ʿillah) is edibility(ṭuʿm) characterized by being suitable for sustenance  (muqtāt) and storability (iddikhār). If edibility alone were the intended factor, it would have sufficed to mention just one of these four categories. However, since the hadith mentioned several categories, it is understood that the intent was to highlight the common underlying feature of each category, which is that they are all items that can be stored and used as sustenance [59] - Refer to: "Bidāyat al-Mujtahid" by Ibn Rushd (3/151). .
Secondly:
The wisdom behind the prohibition of Riba (usury) is to prevent exploitation between people and to protect their wealth. Therefore, this ruling applies to the essential commodities on which people's livelihoods depend, the aqwāt staple foods [60] - Refer to: The previous Reference. .
The Second Issue:
The cause (reason,ʿillah ) of Riba in gold and silver. 
The cause (ʿillah) of usury(Riba) in gold and silver is the mere monetary value (thamanīyah) [61] - The reasoning based on the mere thamanīyah (monetary value) does not limit the cause (ʿillah) to just the two currencies gold and silver; rather, the ʿillah ( cause or reason) extends to other forms of currency that people have adopted and agreed upon. In contrast, if the reasoning is based on the predominance of thamanīyah(monetary value),then the ʿillah would be confined to the two currencies. , which is a position in the Mālikī [62] - "Al-Fawākih al-Dawānī"by Al-Nafrawi (2/74) ; "Hashiyat al-Adawi" on "Kifāyat al-Ṭālib al-Rabbānī" (2/142). school of thought and a narration from Ahmad [63] -"Refer to: "Al-Furūʿ" by Ibn Muflih (6/294) ; " Al-Insāf" by Al-Mardāwi (5/14). . Ibn Taymiyyah [64] -Ibn Taymiyyah said: "The more apparent reasoning (ʿillah,cause,) in this matter is thamanīyah (monetary value), not weight, as stated by the majority of scholars. Therefore, when coins become athmān (prices), they acquire this meaning, and it is not permissible to sell a price for another price with a deferred  payment." "Majmūʿ al-Fatāwā"( 29/471- 472). and Ibn al-Qayyim [65] -Ibn al-Qayyim said: "As for dārāhim (silver coins) and dīnār (gold coins), one group stated that the reasoning (ʿillah,cause)  behind them is their being measured by weight, which is the view of Ahmad in one of the narratives attributed to him, and the view of Abū Ḥanīfah. Another group said that the reasoning behind them is thamanīyah (monetary value), which is the opinion of al-Shāfiʿī, Mālik, and Ahmad in the other narrative, and this is the correct view, indeed, the most accurate." " Iʿlām al-Muwaqqiʿīn"( 2/105). chose this view, and it is the resolution of the Council of Senior Scholars in Saudi Arabia( [66] - In the resolution of the Council of Senior Scholars, it is stated: "The opinion that considers the concept of thamanīyah (monetary value) as a reasoning (ʿillah) for the applicability of Ribā usury in the two currencies is the most evident in terms of evidence and the closest to the objectives of the Sacred Law. This is one of the narratives from the imāms: Mālik, Abū Ḥanīfah, and Aḥmad. Abū Bakr reported that this was narrated from Aḥmad by a group, as it is the preference of some scholars among the learned, such as Shaykh al-Islām Ibn Taymiyyah, his student Ibn al-Qayyim, and others." [ Research of the Senior Scholars, (Abḥāth Hay'at Kibar al-ʿUlamāʾ )(1/92).] , as well as the  Fiqh Council of the Muslim World League [67] -  Has stated: "The Council of the Islamic Fiqh Academy has reviewed the research submitted to it regarding the topic of paper currency and its rulings from a Shari'ah perspective. After discussion and deliberation among its members, it has decided the following: Firstly: Based on the principle that the basis of currency is gold and silver, and that the reasoning (ʿillah) for the applicability of Ribā (usury) in them is the concept of thamanīyah (monetary value), this is the most accurate opinion among the scholars of Shari'ah..." [Decisions of the Islamic Fiqh Council in Makkah Al-Mukarramah - Third Edition p. 113.] . This is also the fatwa of the Permanent Committee "Fatawa al-Lajnah al-Da’imah" in Saudi Arabia [68] -The Fatwa of Permanent Committee stated: "The items in which Ribā (usury) is prohibited are: gold and silver, wheat and barley, dates and salt, as well as anything that shares the same reasoning (ʿillah) of Ribā with these six categories; which is the thamaniyyah (monetary value) in the case of the two currencies, and in the remaining categories, it is kail (measured goods) along with ṭuʿmiyyah (food items), according to the correct opinions of the scholars." [Fatāwā of the Permanent Committee - First Collection 13/268.] .
This is for the following reasons:
Firstly: Because the intended purpose of values is to serve as a standard for wealth, used to ascertain the quantities of money, rather than to seek benefit from the physical items themselves. This leads to a deficiency in the intended purpose of thamanīyah, resulting in harm to the people [69] -  " Majmūʿ al-Fatāwā" - Ibn Taymiyyah (29/471- 472), and  "Hashiyat al-ʿAdawi ʿalā Kifāyat al-Ṭālib al-Rabbānī"( 2/142). .
Secondly: People need a price by which they can evaluate sales is a general and essential necessity. This can only be achieved through a price that defines value, and it must be a price by which things are assessed and remain constant in one condition. If it is evaluated by something else, it would become a commodity subject to fluctuation, leading to the corruption of people's transactions, causing disputes, and increasing harm [70] - "I‘lam al-Muwaqqi‘in" by Ibn al-Qayyim. .
Issue: The status of currency notes concerning gold and silver
Currency notes are considered to have the same ruling as the two currencies, gold and silver; thus, they are subject to both types of usury: excess and deferred [71] -”Mabahith al-Awraq al-Naqdiyya" .
The Second Topic: What is measured by weight or volume in usurious sales
The First Issue: According to the text, what is measured by volume or weight in usurious sales?
What is measured by weight or volume in usurious sales is what the Messenger of Allah صلى الله عليه وسلم has specified, and this pertains to the four types: wheat, barley, dates, and salt—measured by volume; and gold and silver—measured by weight [72] -"It is not permissible to sell a measured item for a measured item of the same kind by weight, nor to sell a weighted item for a weighted item of the same kind by measurement, as stated in the texts." . This is agreed upon by the four schools of Islamic jurisprudence: Hanafi [73] -"Al-‘Inayah" العناية by al-Babarti  (7/14-15) and "al-Bahr al-Ra’iq"  by Ibn Nujaym (6/140). , Maliki [74] -  "al-Taj wa al-Iklil"  by al-Muwāq (4/360); "Hashiyat al-‘Adawi ‘ala Kifayat al-Talib al-Rabbani"  (2/142) ; "Manh al-Jalil"  by ‘Ulaysh  (5/24). , Shafi'i [75] -  "Minhaj al-Talibin"  by al-Nawawi  p. 96; "Mughni al-Muhtaj"  by al-Shirbini( 2/24). , and Hanbali [76] -"Kashaf al-Qina"   by al-Buhuti (3/262). .
The Evidences:
Firstly:  From the Sunnah:
1. On the authority of Abu Sa'id al-Khudri رضي الله عنه the Messenger of Almighty Allah صلى الله عليه وسلم  said: “Do not sell gold for gold, or silver for silver, except in equal weight, equal for equal, and the same for the same.” [77] -   Sahih al-Bukhari (2176) and (Sahih Muslim) (1584) – with the wording being specific to this text. .
2. On the authority of `Ubadah ibn al-Samit رضي الله عنه, the Messenger of Almighty Allah صلى الله عليه وسلم said: "Gold is to be exchanged for gold by weight, and silver is to be exchanged for silver (whether it is row or Dirham) by weight. Barley is to be exchanged for barley by measurement, and wheat is to be exchanged for wheat by measurement. Dates are to be exchanged for dates by measurement, and salt is to be exchanged for salt by measurement. Whoever increases or requests an increase has engaged in usury. It is permissible to sell gold for silver, provided that the greater amount is handed over on the spot. However, selling them on credit is not permissible. Likewise, it is permissible to sell barley for wheat, provided that the greater amount is handed over on the spot, but selling them on credit is not permissible." [78] - It was narrated by Abu Dawood(3349), with the wording attributed to him, and by At-Tirmidhi (1240), An-Nasa'i (4564), and Ahmad(22683). Imam Ahmad regarded it as good, as mentioned in Sunan Ad-Daraqutni (2/590). At-Tirmidhi stated: "it is 'Hasan Sahih' "(Good and Authentic).    Al-Tahawi mentioned in "Sharh Ma'ani Al-Athar"(4/66) that the reports are widespread in this regard. Ibn 'Abd al-Barr, in "Al-Istidhkar"(5/402), stated that it was narrated through numerous channels. Ibn Al-'Arabi said in 'Aridat Al-Ahwadhi (3/200) that it is a fundamental principle of Sharia. Al-Albani authenticated it in "Sahih Sunan At-Tirmidhi"( 1240).Ibn Al-Mulqen authenticated its chain of narration in "Al-Badr Al-Munir"(6/470), and Shu'ayb Al-Arna'ut confirmed it in his verification of "Sunan Abu Dawood (3349). The origin of the Hadith is found in Sahih Muslim (1587).
Reasoning:
The Prophet Muhammad صلى الله عليه وسلم  considered the equality in weights for items measured by weight, and in measures for items measured by volume. Anyone who contradicts this has deviated from the prescribed command, as the equality that is considered in transactions where differentiation is prohibited is the equality mentioned in the Hadith [79] -  Matalib Uli Al-Nuha - Al-Ruhaybani (3/159) , Ihkam Al-Ahkam - Ibn Daqiq Al-Eid p: 370 .
3- On the authority of Ibn 'Umar رضي الله عنهم, he said: The Messenger of Almighty Allah صلى الله عليه وسلم  said: "The weight is the weight of the people of Makkah, and the measure is the measure of the people of Madinah." [80] - This Hadith is reported by Abu Dawud (3340) and narrated with this wording. It is also recorded by An-Nasa'i(2520); Ad-Daraqutni authenticated this Hadith, as cited in At-Talkhis Al-Habir by Ibn Hajar(2/759); Ibn Hazm declared it authentic in Al-Muhalla(11/353)., Ibn Al-Mulaqqin mentioned its authenticity in Al-Badr Al-Munir (5/562)., Al-Albani also confirmed its authenticity in Sahih Sunan Abi Dawud 2520.,  Al-Wadi'i graded it as meeting the conditions of Al-Bukhari and Muslim in As-Sahih Al-Musnad (782);  An-Nawawi confirmed its chain of narrators in Al-Majmu'( 6/2) and said it meets the conditions of Al-Bukhari and Muslim., Shu'ayb Al-Arna'ut authenticated the Hadith in his work on Takhreej Sunan Abi Dawud 3340.
Reasoning:
The statement of the Prophet صلى الله عليه وسلم serves to clarify the rulings; whatever was a measured in Madinah during his time has had its prohibition established concerning the differentiation in measurement. Therefore, it is not permissible for it to change afterward, and the same applies to weight [81] - This is the evidence used by the Hanbali scholars for considering textual proof in determining the measurement of items sold by volume and weight. See: Kashshaf Al-Qina' by Al-Buhuti 3/262. .
Secondly: The text is stronger than customary practices, as it serves as evidence for both those who are familiar with it and those who are not. Custom is not a valid proof except for those who are familiar with it. The stronger evidence should not be abandoned for the weaker [82] -Refer to:  "Al-'Inayah" by Al-Babarti (7/15). .
Thirdly: If something is not defined in Islamic law (Shari'ah) or in the language (Linguistic), then its determination reverts to the customary practices of the people, such as in matters of possession and safekeeping [83] - Refer to: "Al-Mughni Al-Muhtaj" by  Al-Shirbini (2/24). .
The Second Issue: Items governed by measurements (Volume or weight )  in usury Transactions when not explicitly specified:
Items measured by volume or weight in usurious transactions for which there is no explicit textual reference should be determined by what is customary among the people. This is the view of the Hanafi [84] -See: "Al-'Inayah" by Al-Babarti (7/14, 15); " Al-Binayah" by "Al-'Ayni"( 8/275); "Al-Bahr Al-Ra'iq" by Ibn Nujaym(6/140). and Maliki [85] -See: "Al-Taj wal-Iklil" by "al-Muwāq" (4/360); "Hashiyat Al-Adawi 'ala Kifayat Al-Talib Al-Rabbani" (2/142); "Minhaj Al-Jalil" by Ulayish(5/24). schools of thought. This is because anything that lacks a defined limit in Islamic law is referred back to the customary practices that people are accustomed to, and since customs differ from one region to another, each locality should adhere to the practices established by its inhabitants [86] -  "Al-Taj wa al-Iklil" by al-Mawwaq(4/360); "Hashiyat al-Adawi"(2/142); and "Al-Bahr al-Raiq"by Ibn Nujaym(6/140). .