Muwatta 1893

Malik says of a man who is owed food by another, which he bought from him, while his creditor is owed food of the same kind by another man: the debtor tells his creditor: I refer you to a debtor of mine who owes you food like the food that you are owed by me, in exchange for your food that you are owed by me. Malik said: if the debtor's food is food that he bought, and he wants to refer his creditor to it, that is not proper, and that is selling food before taking possession. But if the food is an advance, due now, there is no harm in referring his creditor to it, because that is not a sale, and it is not permitted to sell food before taking possession, since the Messenger forbade that. ¹

Belongs to the Muwatta, Book of Sales. Source: the Muwatta. Mentions Malik ibn Anas. Built on in the chapters on sale and advance purchase (salaf).