Is it permissible to sell a processed derivative of milk that has been mixed with a non-milk substance (like Kashk or Kamikh) for the original milk or another derivative?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The sale of milk derivatives mixed with non-milk substances, such as Kashk or Kamikh, for the original milk or any other derivative is not permissible because of the admixture, placing it in the same category as the complex usury case involving dates. However, if the admixture (in products like Mukhid, Lab'a, cheese, etc.) is for the benefit of preserving the derivative itself, then it is permissible to exchange one type for another of the same processed type if they are equal in dryness and moisture content. Examples include sour milk for sour milk, cheese for cheese, and dried milk solids (Aqit) for dried milk solids, provided they are equal in terms of measure (kil or weight).
Supporting text
Equality between Aqit, Mصل (Mussal), and Mukhid must be assessed by measure (kail) because they are quantified similarly to the measure used for Zakat al-Fitr. Cheese, butter, and ghee are to be sold by weight as they are weighed commodities which cannot be measured by volume, similar to bread. A dry version of these products cannot be exchanged for a moist version, just as fresh dates cannot be exchanged for dried dates (Tamar).