Preliminary field of research: Milk as Different Categories and Its Ribawī Status
The First Issue: Milk is of Different Categories.
Milk is classified into different categories based on its sources. Sheep’s milk is one category, camel’s milk is another, and cow’s milk is yet another. This is the view of the majority of scholars: the Ḥanafī [214] -'Al-Bināya by Al-ʿAynī (8/294), Al-Fatāwā al-Hindiyya (3/120) , Shāfiʿī, [215] - "Fatḥ al-ʿAzīz " by Al-Rāfiʿī (8/185); "Rawḍat al-Ṭālibīn" by Al-Nawawī (3/395);       "Mughnī al-Muḥtāj" by Al-Shirbīnī (2/24). and Ḥanbalī schools [216] - "Kashshāf al-Qināʿ "by Al-Buhūtī (3/255); "Maṭālib Ulī al-Nuhā" by Al-Ruḥaybānī (3/161). . This is because they are derived from distinct sources, making them different categories by analogy to their origins [217] -"Mughnī al-Muḥtāj "by Al-Shirbīnī (2/24);  "Kashshāf al-Qināʿ"  by Al-Buhūtī (3/255). .
The Second Issue: Milk is a Ribawī Commodity
Milk is considered one of the Ribawī commodities, as it is both consumed and measured by volume. The rulings on selling milk for milk will be discussed in the following sections.
The First Topic: Selling Milk for Milk of the Same Category
It is permissible to sell milk for milk of the same category if the transaction is done hand to hand and is equal in quantity. It is not permissible to do so with disparity in quantity or on deferred payment terms. The four schools of Islamic jurisprudence agree upon this: Ḥanafī [218] -"Tabyīn al-Ḥaqāʾiq "by Al-Zaylaʿī with the commentary of Al-Shilbī (4/94),  "Al-Baḥr al-Rāʾiq "by    Ibn Nujaym (6/146). , Mālikī [219] - "Al-Tāj wa al-Iklīl"  by Al-Mawwaq (4/353); " Mawāhib al-Jalīl"  by Al-Ḥaṭṭāb (6/210). , Shāfiʿī [220] - "Mughnī al-Muḥtāj" by Al-Shirbīnī (2/24); "Ḥāshiyat Qalyūbī wa ʿAmīrah" (2/211). , and Ḥanbalī [221] -  "Al-Inṣāf" by Al-Mardāwī (5/18), "Kashshāf al-Qināʿ "by Al-Buhūtī (3/255) . This is because milk of the same category falls under the ribā regulations, and thus cannot be sold with disparity or deferred, just like selling dates for dates [222] - See: "Al-Muhadhdhab"  by Al-Shīrāzī (2/38). .
The Second Topic: Selling Milk for Milk of Different Categories.
It is permissible to sell milk for milk of different categories with disparity, provided it is done hand to hand. This is the view of the majority of scholars: Ḥanafī [223] -"Tabyīn al-Ḥaqāʾiq" by Al-Zaylaʿī with the commentary of Al-Shilbī (4/94); "Al-Baḥr al-Rāʾiq" by Ibn Nujaym (6/146). , Shāfiʿī (according to the more correct view) [224] - "Mughnī al-Muḥtāj" by Al-Shirbīnī (2/24); "Ḥāshiyat Qalyūbī wa ʿAmīrah" (2/211). , and Ḥanbalī schools [225] - "Al-Inṣāf" by Al-Mardāwī (5/18), "Kashshāf al-Qināʿ "by Al-Buhūtī (3/255). . The reasoning is as follows:
Firstly, their sources are considered different categories, such that they are not combined for zakāh purposes, and their names also differ based on their addition (e.g., wheat flour and barley flour). The intended purpose of consumption and strengthening is also different, as some people prefer one type over another, and one may be harmful while the other beneficial. The criterion for unity is considered in the specific meaning, not the general one. If the general meaning were considered, it would never be permissible to sell one thing for another [226] - This is evidence that milk consists of different types." Tabyīn al-Ḥaqāʾiq" by Al-Zaylaʿī (4/94),  "Mughnī al-Muḥtāj" by Al-Shirbīnī (2/24). .
Secondly, milk is a byproduct of its source, and byproducts of different categories are themselves considered different categories. When the categories differ, the disparity is permitted, but deferred payment is prohibited [227] -"Al-Furūʿ" by Ibn Muflīḥ (6/299), "Kashshāf al-Qināʿ "by Al-Buhūtī (3/255). .