The First Topic: Definition of the Sale of al-'Iyna(العينة) Linguistically and its Islamic (Shari'a) meaning:
· Linguistic Definition of al-'yIna(العينة): It refers to a deferred payment. It is said: "A person has engaged in al-'yIna(العينة) when he buys something in exchange for another thing on credit [320] -See: "Tāj al-'Arūs" by al-Zabaīdī (35/458) . It is derived from the term "ayn," which refers to ready cash. This sale is called al-'yIna(العينة) because the buyer of the item on credit takes cash in return [321] -See: "al-Zāhir" by al-Azharī (p. 142). .
· Islamic (Shari'a) Definition of the Sale of al-'yIna(العينة): It is when a person sells something to another for a deferred price, delivers it to him, and then repurchases it before receiving the payment for a lower price in cash [322] - See: Sharh Hudud ibn 'Arafah (p.266); Sharh Muntaha al Iradat by al-Buhuti (2/25); al-Sharh al-Mumti' by Ibn 'Uthaymin(8/238). .
The Second Topic: The Ruling on the Sale of al-'yIna(العينة).
The sale of al-'yIna(العينة) is prohibited [323] - Scholars have mentioned several forms of this transaction, including: 1.    Selling a commodity for a deferred price, and then repurchasing it from the buyer for a lower cash price before the due date. This is the well-known form that the majority of scholars have explicitly forbidden. 2.    Selling it for a deferred price and then buying it back from the buyer for a higher price with a new, later due date. 3.    Involving a third party in the transaction, where the lender sells a garment, for instance, to the borrower for twelve dirhams and delivers it to him. Then the borrower sells it to the third party for ten dirhams and delivers it to him. The third party then sells it back to the lender (the original owner) for ten dirhams, delivers it, and the lender takes the ten dirhams and gives them to the borrower. Thus, the borrower gains ten dirhams, and the original owner of the garment gets twelve dirhams. 4.    Selling a commodity for cash and then repurchasing it for a higher deferred price. See: "Hashiyat Ibn Abidin" (5/273);" al-Kafi " by Ibn Abd al-Barr (2/671); "al-Mubdi‘ " by Burhan al-Din Ibn Muflih (4/49);, "Majmu' al-Fatawa" by Ibn Taymiyyah (29/441). , and this is the opinion (view) of the majority [324] -This applies when the second sale is not stipulated in the contract. However, if the second sale is a condition in the contract, the jurists have unanimously agreed on its prohibition.   See: "al-Muhalla" by Ibn Hazm (7/548); "Al-Majmu' Sharh al-Muhadhdhab"; "Takmila al-Subki" (10/157).  Similarly, it is permissible for a person to sell something to another for a deferred price, deliver it to him, and then buy it back from him for the same price or more.   See: "Bada'i al-Sana'I" by al-Kasani (5/199);  "Mawahib al-Jalil" by al-Hattab (6/293);"Al-Mughni " by Ibn Qudama (4/132). : Hanafis [325] -See: "Tabyin al-Haqa'iq" by al-Zayla'i (4/53); "al-Bahr al-Ra'iq" by Ibn Nujaym (6/90) , Malikis [326] -See: "Al-Taj wa al-Ikleel "by al-Mawwaq (4/388); "Mawahib al-Jaleel" by al-Hattab (6/293). , and Hanbalis [327] -See:  "al-Mubdi' " by Burhan al-Din Ibn Muflih (3/387); " al-Insaaf" by al-Mardaawi (4/242). , and it is also the view of a group of the Companions and Tabi'in [328] -Burhan al-Din Ibn Muflih said: "And this is the opinion of a group of the Companions and those who followed them." See: "al-Mubdi' by Burhan al-Din Ibn Muflih" (3/387). .
The Evidences:
Firstly: From the Sunnah:
On the authority of Ibn 'Umar (رضي الله عنهما), he said: “I heard the Messenger of Allah صلى الله عليه وسلم say: ‘When people become stingy with the dinar and dirham, engage in al-'yIna(العينة) transactions, follow the tails of cattle (Worldly preoccupied), and abandon jihad (Holy war in the way of Allah), Allah will inflict upon them a calamity, and He will not remove it until they return to their religion. [329] - ((It was narrated through various chains by Abu Dawud (3462) with similar wording, and Ahmad (4825) with this specific wording.))      Al-Tabari authenticated its chain in "Musnad ʿUmar" (1/108);and Ibn Taymiyyah authenticated its chain in "Bayān al-Dalīl" (109); as did Ibn al-Qayyim in  " Iʿlām al-Muwaqqiʿīn" (3/143); Ahmad Shakir in"Musnad Ahmad"(7/27); and Ibn Baz said:"the Hadith is Hassan Lighairihi (حسن لغيره) "  in ((Ḥāshiyat Bulūgh al-Marām)) (504). Al-Albani authenticated it in " Ṣaḥīḥ Sunan Abī Dāwūd" (3462) and in "Ṣaḥīḥ al-Jāmiʿ " (675); and Al-Wādiʿī considered it Hasan (sound) by considering its various chains in "Ṣaḥīḥ Dalāʾil al-Nubuwwah" (559). ’”
The expression of the Prophet Muhammed صلى الله عليه وسلم  (follow the tail of cattle) means that Muslims being preoccupied with agriculture, trade and worldly pursuit instead of doing very hard for Islamic religion and striving  in the path of Allah.
Jihad is Holy fighting in the Cause of Allah or any other kind of effort to make Allah's Word (i.e. Islam) Superior. Jihad is regarded as one of the fundamentals of Islam.
Reasoning:
His statement: “Allah will inflict upon them a calamity, and He will not remove it until they return to their religion” is an indication of the prohibition of al-'yIna(العينة), as it is followed by a threat of humiliation. Seeking the causes of religious dignity and avoiding the causes of humiliation, which contradict the faith, are obligations upon every believer. The threat of calamity suggests that such an act is a severe sin, and it places the one who engages in it in the position of someone who has left the religion [330] - See: "Nayl al-Awṭār" by al-Shawkānī (5/246) .
Secondly: From the Athar (Reports from the Companions and their students)
(Athar: is a saying, ruling or tradition of the Prophet's Companions and their students.)
On the authority of Ibn 'Abbas (رضي الله عنهما ), regarding a man who sold a silk garment on deferred payment and then bought it for less than what he sold it for, in cash, he said: "This is like exchanging dirhams for dirhams, with the silk garment in between them. [331] - It was narrated by Ibn Abī Shaybah in "al-Muṣannaf" (20157), and by Ibn al-Mundhir in "al-Awsaṭ" (8165) with this specific wording, and by Ibn Ḥazm in "al-Muḥallā" (9/106).= =Ibn al-Qayyim affirmed its authenticity in "Tahthīb Sunan Abī Dāwūd" (2/142), and Ibn ʿUthaymīn confirmed it in "Majmūʿ al-Fatāwā" (181/28). "
Thirdly: Because it is a means to Riba (usury); it allows one to sell one thousand for five hundred on deferred payment. And the means (that lead to sin) are considered in the Shariah, as evidenced by the prevention of the murderer from inheriting [332] - See: "al-Mubdiʿ " by Burhān al-Dīn Ibn Mufliḥ (3/387). .
Fourthly: Because Allah rebuked the Children of Israel for using legal tricks (hiyal) to engage in what they were forbidden [333] -See: ((the previous source)) .
Fifthly: Because Allah Almighty prohibited Riba, and al-'Iyna(العينة) is a means to Riba(Usury), and the means to haram(forbidden) are also haram [334] - See: "ʿAwn al-Maʿbūd" and  "Ḥāshiyat Ibn al-Qayyim" (9/241). .