Is it permissible to collect one type of currency owed (e.g., dirhams) against another type owed (e.g., dinars) by settling at the prevailing market rate?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The majority of scholars permit collecting one currency against another, viewing it as a form of exchange involving a physical asset and a debt claim (Sarf bi 'Ayn wa Dhima). This is supported by the narration of Ibn Umar, where the Prophet, peace be upon him, allowed taking the equivalent value based on the day's price provided the parties did not separate with anything remaining unsettled between them. Ahmad stated that the payment must be made according to the day's price, equating value due to the impossibility of physical likeness when the currency differs.
Supporting text
Ibn Abbas, Abu Salama ibn Abd al-Rahman, Ibn Shubruma, and a reported view from Ibn Mas'ud prohibited this, arguing that immediate possession ('qabd') is a condition for exchange, which is missing here since the debt is in the debtor's possession. Some scholars permitted the exchange only by mutual consent based on the market price, analogous to a present sale.