Do not sell what you do not have
Question: Today, at some commercial places, when a buyer goes to purchase some goods, the salesman says: Please wait a little. Then he goes to another shop and brings that item. What is the ruling on this situation? Does this transaction fall under bay’ salam or not?
Answer: If they both conclude the sale contract, then it is not permissible, because the Prophet ﷺ said: “Do not sell what is not with you.”
But if they make a promise and the shopkeeper says: Come after Asr, the goods were requested by him in the morning, but the shopkeeper intends that he will buy this item and sell it to him after Asr, then there is no harm in it, because the sale contract has not taken place; only a promise to supply the goods has been made.
The important point is that before the goods arrive, there should be no agreement between the two, and in the promise they should not bind each other, but only say: In sha Allah, I will bring it for you in the evening. If the prospective buyer buys those goods from another place, then this shopkeeper should not say to him: Why did you buy these goods?
