Is the method of installment sale, which involves purchasing the commodity after the buyer's approval and then handing him the invoice so he can receive or sell it, prohibited? And what is the correct method for installment sale?

This transaction is forbidden for the following reasons: 1. Selling the commodity before it is moved and possessed, based on the prohibition by the Prophet, peace and blessings be upon him, against selling commodities where they are sold until merchants take possession of them and move them to their conveyances. 2. Profiting from something that has not come under one's guarantee, based on the Prophet's prohibition against profiting from something that has not been guaranteed. 3. It might be a form of the forbidden bay' al-inah (a type of buy-back sale), which is a stratagem to circumvent usury, where the buyer sells it back to the first party who sold it to you.

Those who permit this transaction stipulate that the commodity must be taken possession of, moved, and be under the seller's guarantee, and that it should not be sold before it is owned. In this transaction, the first condition is missing.