Is it permissible to engage in an installment transaction with a person who appoints his sister to carry out the purchase in order to avoid direct dealing in the purchase, knowing that he is the beneficiary and will pay the amount with interest, so as to avoid the suspicion of usury?

It is permissible for a customer to ask a merchant to purchase a specific commodity for them to buy later (murabaha sale to a purchasing agent), provided that the merchant does not sell the commodity before acquiring ownership and possession of it (removing it from the first seller's premises), and that the contract is free from a penalty clause for late payment. A bank is prohibited from appointing a customer as an agent for purchase except in the narrowest of circumstances; the norm is for the bank itself to take possession of the commodity. The merchant, acting as a purchasing agent, buys the commodity himself or appoints someone other than the customer. If this is not feasible, he may appoint the customer, and the commodity then remains a trust in the agent's possession until it is actually sold to him, and it should not be sold while still at the first merchant's premises. Loss or destruction without transgression or negligence is borne by the principal. It is preferable for the merchant to purchase the commodity himself or to appoint someone other than the customer to avoid any suspicion and to steer clear of usury.