Is it permissible to trade a derivative subject to Riba for the same genus of that derivative?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The exchange of a derivative product for the same genus of that derivative product is permissible, provided the exchange is equal in measure (tamāthulan). If the exchange is for a different genus, it is permissible to trade unequal amounts (mutafāḍilan) in any manner desired because they constitute two distinct genera. Equality is determined by the standard measure (kil, volume) used for buying and selling these items. This is the established doctrine of Al-Shafi'i, and this ruling applies whether the items are cooked or raw.
Supporting text
Some followers of Al-Shafi'i argue that trading cooked items of the same genus is not permissible because the heat causes their components to coagulate, leading to variation and potential disparity in measure. However, the ruling stands that they are considered equal in their current state without one having an inherent deficiency.