Is it permissible to trade an item subject to Riba (usury) for its primary derivative?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
It is impermissible to trade any item subject to Riba for its principal source from which it is derived. This prohibition applies to sesame seeds exchanged for sesame oil (Shīrj), olives for olive oil, other oils for their source materials, and grape, pomegranate, apple, quince, or sugarcane juice exchanged for the original fruit or cane. This ruling is held by Al-Shafi'i and Ibn al-Mundhir. The evidence for this is derived from the principle that an item subject to Riba cannot be exchanged for its source from which the item itself originates, similar to exchanging meat for the living animal, a principle established by clear text.
Supporting text
Abu Thawr permits such an exchange because the origin and the derivative are distinct entities. Abu Hanifa permits the exchange if it is known with certainty that the quantity of oil or juice in the source material is less than the separated derivative; otherwise, it is impermissible.