Is it permissible to exchange one separated derivative of milk for another separated derivative (e.g., butter for sour milk)?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The exchange of butter or ghee for sour milk is permissible, both equally and differentially. This is because butter and ghee are considered two distinct species originating from one source, likened to meat and fat. Furthermore, the small amount of milk remaining in butter is considered insignificant, similar to salt in sesame oil. The exchange of ghee for sour milk is considered more permissible due to ghee being devoid of sour milk entirely.
Supporting text
The sale of butter for ghee is forbidden because butter retains a small amount of milk residue while ghee does not, thus destroying equality. Furthermore, ghee is extracted from butter, so selling the derivative for the derived substance is invalid, similar to selling olives for oil. The opposing view holds that the small, unintentional presence of milk in butter is negligible, validating the exchange, but this is rejected because the presence of milk in one and its absence in the other violates the requirement for parity, akin to trading pitted dates for unpitted dates.