If a person owes another a specific quantity of gold coins (dinar) and repays it gradually using silver coins (dirhams), is this valid?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the debtor pays the dirhams corresponding to the value of the dinars owed for each installment, the transaction is valid. Ahmad explicitly stated this ruling. If the debtor does not adhere to this per-installment valuation and only settles the amounts after calculation later, the transaction is impermissible. This is because when the dirhams are later exchanged for the dinar debt, it becomes a sale of debt (the dirhams becoming debt) for a debt (the dinar), which is prohibited.
Supporting text
If the debtor pays the dirhams gradually without immediate valuation, and later they are valued to settle the dinar debt, the valuation applies to the day of final settlement, not the day the dirhams were initially handed over. This is because the dirhams were merely a deposit in the recipient's possession prior to valuation, and their loss or damage liability is primarily with the owner, though another view suggests liability falls on the recipient since they were received intending settlement.