Is it permissible for a Muslim to help his brother purchase a used car in installments without a down payment, with the addition of a small profit margin (interest) to the original price, of which the buyer is aware and content? If the answer is no, what are the conditions for permissible installment payments with a lawful profit?

Installment selling is permissible in principle. There is no harm in increasing the price of a commodity sold on credit, or above its immediate cash price, by agreement of the jurists. It is stipulated that the contract between the seller and the buyer must be for a specific, known price. More than one scholar has narrated the consensus of the 'ulama' on the permissibility of installment selling.