Is the sale of a sheep possessing milk for its own milk permissible, or a sheep possessing wool for its own wool, or a lactating sheep for another lactating sheep?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
There are two opinions regarding the exchange of a sheep with milk for milk, or a sheep with wool for wool, or a lactating sheep for another lactating sheep. The first opinion asserts permissibility, favored by Ibn Hamid and being the view of Abu Hanifa. This permission applies whether the sheep is alive or slaughtered, based on the principle that the Riba-susceptible element is incidental and thus does not invalidate the exchange, analogous to the overlaid roof on a house. If the sheep is currently milked, its sale for its like or for milk is permissible unanimously, because the milk is inconsequential and not accounted for in the price, similar to salt in sesame oil paste (Shirez), bread, cheese, or barley grains in wheat. There is no dispute on this point. Furthermore, if the milk exchanged is of a different genus than the sheep's milk, the exchange is permissible under all circumstances.
Supporting text
The second opinion dictates prohibition, which is the madhab of Al-Shafi'i. This is because one is exchanging a Riba-subject item for its origin which contains that Riba-subject element, analogous to exchanging an animal for its meat. The distinction noted is that meat within an animal is intended, unlike milk. The reason for the minority view on prohibition is that the fruit (like dates) can be sold separately and is known, unlike the milk from a sheep; however, this distinction is considered ineffective, as what causes prohibition invalidates the exchange even if it can be sold separately, such as a jeweled sword sold for its gems, and what is permissible does not invalidate the exchange even if it can be sold separately, such as a slave's wealth.