Is it permissible to sell milk derivatives (like butter, ghee, sour milk) extracted from milk back for the original milk?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The sale of extracted milk products such as butter (Zubd), ghee (Sمن), sour milk (Mukhid), and colostrum (Lab'a) for the original milk is forbidden. This is prohibited because the derivative is extracted from the original substance, making it impermissible to exchange it for its base product, similar to selling an animal for its meat, or sesame seeds for sesame oil. This is the established position of the Shafi'i school.
Supporting text
There is an opinion attributed to Ahmad stating that it is permissible to sell milk for butter if the separated butter exceeds the amount of butter contained within the milk. This implies that differential exchange might be allowed, but equal exchange is forbidden. However, this ruling is considered outside the main established school position, as commodities subject to usury (Riba) cannot be exchanged for themselves mixed with a non-genus item.