Muwatta 1857

Malik says: if a man exchanged dirhams for a dinar, then finds a counterfeit dirham in them and wants to return it, the exchange of that dinar is broken: he returns the silver to him and takes his dinar back. The explanation of what is disliked in this: the Messenger said: gold for silver is riba unless hand to hand. Umar ibn al-Khattab (may God be pleased with him) said: if he asks you to wait until he enters his house, do not wait for him. If he returns a dirham from the exchange after they have separated, it is like a debt or a deferred item, so that was disliked and the exchange was broken. Umar meant that gold, silver and all food should not be sold immediately for deferred payment. ¹

Belongs to the Muwatta, Book of Sales. Source: the Muwatta. Mentions Malik ibn Anas. Built on in the chapters on currency exchange (sarf) and riba.