Is it permissible for two persons, each having a claim (debt) against the other involving different currencies (gold owed for dirhams owed), to settle these reciprocal debts simultaneously?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The settlement of reciprocal debts involving different currencies due between two parties is impermissible. This view is held by Al-Layth and Al-Shafi'i. The basis for prohibition is that this constitutes a sale of debt against debt (bay' al-dayn bi al-dayn), which is impermissible by scholarly consensus. Ibn Al-Mundhir affirmed that the scholars unanimously agreed that selling debt for debt is prohibited, and Ahmad considered it established consensus. Evidence for this prohibition includes the narration that the Prophet, peace be upon him, forbade the sale of deferred asset for a deferred asset (bay' al-kali' bi al-kali'), which is interpreted as debt for debt.
Supporting text
A narration attributed to Malik and Abu Hanifa, conveyed by Ibn Abd al-Barr, permits this exchange. Their reasoning is that a present debt claim is analogous to a present physical asset, permitting transactions like exchanging dirhams for unspecific dinars without prior designation.