What is the ruling regarding an office owner selling goods to a customer in installments, while he (the office owner) purchases them in cash from a third party, and then the customer, in turn, sells them directly to the third party while they are still in their place? And what is the legitimate alternative to that?

If the first party sells the merchandise before taking possession of it, the contract is invalid regarding food, by consensus, and regarding other goods, according to the preponderant opinion. However, if he takes possession of it before selling it, the contract is valid, and the second party has the right to dispose of his goods, and he is permitted to engage in Tawarruq. It is preferable for the second party to avoid selling the item to a third party to distance oneself from the suspicion of usury. If there is collusion to that effect, it would be considered prohibited usury.