Is it permissible to conduct a transaction for purchasing a computer in installments from an agency that does not own the product at the time of the contract, but rather buys it after reaching an agreement with the buyer from another seller, knowing that the buyer is the one who goes to the original seller to obtain the price and then receives the product directly from him? And is it permissible to make use of the device if the transaction is impermissible?

If the agency does not first buy the computer for itself and then sell it to you, but rather pays the price on your behalf and then recovers more than it paid from you at a later date, then this is a prohibited usurious transaction, and it must be فسخ (dissolved/annulled) if possible; otherwise, one must seek forgiveness and repent. However, if the agency buys the computer for itself and then sells it to you as a murabaha (cost-plus profit) after it comes into its possession, this is permissible, provided that you take possession of the computer from the company as an agent for the agency, and then take possession of it from yourself for yourself.