What is the ruling when one party receives a surplus beyond the agreed-upon currency amount during an exchange confirmed as 'one Dinar for one Dinar'?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the phrasing was 'I sold you one Dinar for one Dinar' and they took possession, the surplus amount held by the receiver is considered undivided property, guaranteed to the owner. This is because the surplus was received under the assumption of it being counter-value, and the primary contract itself (Dinar for Dinar) is not voided. The recipient may return an equivalent value for the surplus, whether of the same currency or a different kind, as this constitutes a new, separate transaction.
Supporting text
If the initial phrasing was 'I sold you this Dinar for this [other amount],' the contract is void because it constitutes selling gold for unequal gold. If either party wishes to void the transaction after the surplus is discovered, they have the right, as the recipient found the exchanged item mixed with another, defective by the defect of partnership (company). The payer is not obligated to accept compensation unless it occurs within the same sitting (majlis), in which case the surplus must be returned, and its proper replacement delivered.