What is the ruling on a promise to purchase, and is it considered usury (riba)?

A promise to purchase is not a purchase. If a person asks his brother to buy an item and then sell it to him, there is no harm, provided that the purchase is completed and possession is obtained first, based on the Prophet's (peace be upon him) saying: "Do not sell what you do not possess."

Similarly, it is not permissible to sell goods where they are bought until merchants take possession of them and move them to their own premises. Nor is it permissible for the second buyer to sell the item until he transfers it into his possession and to another location, based on the hadith from Ibn Umar: "We used to be struck during the time of the Messenger of Allah (peace be upon him) for selling food at its place of purchase until we transported it to our own premises."