What is the ruling on exchanging foodstuffs containing non-identical added ingredients that are not intended for their own sake (like bread or starch)?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
When the added substance is unintentional and added for the sake of the primary substance, such as bread or starch, it is permissible to exchange one item for another of the same type if they are equal in dryness and moisture. Equality must be established by weight because it is the customary measure, as measuring by volume (kail) is not possible. This view is supported by the general principle that a measured or weighed staple commodity prohibits unequal exchange due to excess (riba). Equality must be ascertained using the established religious standard, just as wheat is exchanged for wheat. The permission to exchange them while moist is based on the fact that their main utility is in the moist state, analogous to milk exchanged for milk. The fact that one item may yield more heat upon drying than the other while moist is overlooked if the difference is minor and unavoidable, similar to exchanging new grain for old.
Supporting text
Imam Malik, Al-Awza'i, and Abu Thawr permit the exchange if one party strives to ensure equality without weighing. A narration attributed to Abu Hanifa suggests it is permissible to exchange one loaf for two loaves. Imam Al-Shafi'i holds that exchange is impermissible unless the item is dried and ground finely to be sold by measure, as it is a measurable item where equality is required but measurement is currently infeasible. He also argues that since each contains a non-native substance, exchange is forbidden, like mixing substances in gold or silver.