What is the ruling on exchanging commodities where equivalence (*tamāthul*) is not required, such as different genera or items exempt from usury?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
When equivalence is not a requirement, such as when dealing with different genera or items not subject to usury restrictions, it is permissible to sell one part for another by measure, weight, or by estimation (*juzāf*). This view is held by the majority of scholars, based on the Prophet's permission: "If the two types differ, then sell as you wish," which applies when the genus differs. The explicit prohibition against selling a measured commodity for a measured commodity by estimation is restricted to commodities of the same genus where equivalence is mandated.
Supporting text
A group of our jurists restrict the sale of measured goods for measured goods by estimation, and weighed goods for weighed goods by estimation, even if the genera differ. Imam Ahmad, in one narration, expressed dislike for this. Some scholars restrict the sale of measured goods for measured goods by estimation, or weighed goods for weighed goods by estimation, regardless of whether the genera agree or differ, citing the prohibition against exchanging food for food by estimation, unless one is measured and the other weighed.