Is it permissible to take compensation for a defect when exchanging currency of different types?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the currency exchange involves different genera, taking compensation for a defect during the transaction session (Majlis) is permissible because the requirement of equality is not enforced. Furthermore, any temporary disparity in receiving one consideration before the other within the session is negligible and permissible, similar to other sales transactions.
Supporting text
If the exchange occurs after the parties have separated (Tafarruq), taking the Arsh is impermissible because it results in separation occurring before the full receipt of one consideration. An exception exists if the parties stipulate that the Arsh itself should be of a different type than the currency being exchanged, such as taking a measure of wheat as compensation for a defect in silver currency, which is then permissible even after separation. This principle applies to all riba-governed items requiring immediate exchange; if the Arsh taken is not one of the items for which immediate receipt is stipulated (e.g., taking a Dirham as compensation for a defect in wheat when the exchange was wheat for barley), it is permissible even after separation because separation did not occur before the receipt of the item requiring immediate exchange.