Praise be to Allah.
A “promise to buy” is not a purchase, but it is promise to do so. If a person wants to buy something and asks his brother to buy it then sell it to him, there is nothing wrong with that, if he buys it then takes possession of it, then sells it after that to the one who wants to buy it, because it says in the saheeh hadeeth that was narrated by Hakeem ibn Hizaam (may Allaah be pleased with him) that he said: O Messenger of Allaah, a man comes to me wanting to buy an item that I do not have, so I sell it to him, then I go and buy it.” The Prophet (peace and blessings of Allaah be upon him) said: “Do not sell what is not in your possession.” This indicates that if he sells it to his brother after he has taken possession of it and it has become his, there is nothing wrong with that.
Similarly, the Prophet (peace and blessings of Allaah be upon him) said in a saheeh hadeeth narrated by ‘Abd-Allaah ibn ‘Amr ibn al-‘Aas (may Allaah be pleased with him): “It is not permissible to give a loan and sell at the same time or to sell what is not in your possession.” And it is proven in the hadeeth of Zayd ibn Thaabit (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) forbade selling items where they had been bought, until the merchants moved them to their own place.
From the ahaadeeth that we have quoted, it is known that if a person finds an item with Zayd or ‘Amr (i.e., with any person), such as a car or grains or clothes or vessels and so on, there is nothing wrong with him buying it and acquiring it and taking possession of it, if the seller has completed the purchase procedure and has taken possession of it. But the second seller should not sell it until he has moved it to another place, to his house or to the market, and has taken it out of the seller’s place to another location, then he may sell it after that if he wants, in accordance with the ahaadeeth quoted, and in accordance with the report narrated by al-Bukhaari in his Saheeh from Ibn ‘Umar (may Allaah be pleased with him) who said: At the time of the Messenger of Allaah (peace and blessings of Allaah be upon him), we used to refrain from selling food until we had moved it to our own place.” According to another version: “Until we had moved it from the top of the market to the bottom, or from the bottom to the top.” And Allaah is the source of strength. End quote.