Is it permissible to exchange different types of flour (Daqiq) or parched barley flour (Suwayq) for each other?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The exchange of one type of flour for another type, or one type of parched barley flour for another type, is permissible provided they are equal in measure. This is the opinion held by Abu Hanifa. The evidence supporting this permissibility is that they are equal at the time of the contract in a manner where neither can be singularly deficient, analogous to exchanging dates for dates. If this is established, the exchange must be by measure (kail), because wheat (from which flour is derived) is measured, and flour and parched barley flour have not adopted a different standard. A condition for validity is that they must be equal in fineness (smoothness). If they differ in fineness, it becomes akin to exchanging wheat for flour.
Supporting text
The established view attributed to Al-Shafi'i prohibits this exchange. This is because Al-Shafi'i requires their equality at the stage of completeness, which is when they are still wheat. Since that stage is past, and one flour might come from a heavier wheat grain while the other comes from a lighter one, even if they are equal as flour, they are not equal as wheat.