Muwatta 1892

Malik says: whoever buys food at a known price for a stated term, and when the term falls due the one who owes the food says to his creditor: I have no food, so sell me the food you are owed for a later term. The owner of the food says: this is not proper; the Messenger forbade the sale of food until it is taken. The debtor then says to his creditor: sell me food on a term, and I will pay you, which is not proper either, because he gives him food and then returns it to him. Then the gold he gave becomes the price of the food that was owed to him, and the food he gave becomes permitted between them. If they do this, it is the sale of food before it is taken possession of. 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).