The ruling on selling something before buying it

Question:
May Allah reward you. This is a question regarding transactions. This questioner asks: Honorable Shaykh, what is the ruling on a person who comes to a merchant and says, “Will you buy from me a certain quantity of water at a specific price?” That person does not own that merchandise, but he goes and buys it from a wholesaler at a price lower than the price agreed upon with that merchant. If that merchant agrees to the purchase, the person goes to the wholesaler, buys it from him, and then sells it to that merchant.

Answer:
As for when this concerns specific goods, such as saying, “I will buy you that specific car, that specific house, or that specific piece of land,” a specific item that he does not own, this is not permissible. This is based on the saying of the Messenger (peace and blessings be upon him) . Since he is selling him a specific item that is in the possession of another, then he goes and buys it and delivers it to the one with whom he contracted, this is forbidden. The saying of the Messenger (peace and blessings be upon him): “Do not sell what you do not have.”
But if he sells him goods that are not specific, but are described and secured by the description in the debt, yes, this is valid on the condition that the entire price is paid in the session of the contract. Because this has the ruling of a salam (forward sale) debt. It is not valid unless the entire price is paid in the session of the contract.
Then the substitute is established in the debt, in the debt of the one to whom it is sold, and he goes and works to obtain it, then delivers it to its owner when the time comes, when the term of the debt arrives. But if the price is not paid in the session of the contract, it is not valid, because it becomes a sale of debt for debt, and the Prophet (peace and blessings be upon him) forbade selling gold for gold.