What is the ruling on returning the remainder of the price of a commodity with a currency other than the one it was paid with, such as if the seller is given 100 dollars and then returns the remainder (10 dollars) in another currency?

If the seller and buyer agree that the price of the commodity is in US dollars, and the buyer has a remaining balance with the seller:

1. First case: They agree to pay the remaining balance in another currency. This requires immediate possession (qabd) in the same session before they part ways. If they part ways before that, the exchange is invalid. 2. Second case: The seller does not have change (small denominations), neither in dollars nor in another currency. In this case, it is permissible to leave the remaining balance as a trust with the seller, provided that it remains owed by him in the same currency (dollars). When the remaining balance is claimed, they have the right to agree to pay it in the same currency or in another currency, and the exchange (currency conversion) takes place at the time of payment, not when the trust is left; this is to prevent delaying possession in currency exchange.