Is it permissible to sell a commodity by installments after agreeing with the customer on its cash and installment price and specifying the payment period, knowing that the seller does not own the commodity at the time of the agreement, but rather buys it with his own money from the market after the agreement and then sells it to the customer, without taking any down payment or advance from the customer?

If the seller purchases the goods and they enter into his possession and responsibility before selling them to the buyer, there is no objection to that. This does not fall under the category of selling what one does not own, because what precedes the purchase is merely a promise. It is permissible to request a "seriousness margin" from the buyer, which is an amount he pays to guarantee his seriousness in purchasing. A portion of this amount is deducted to cover damages if the buyer defaults, and if the purchase is completed, this amount becomes part of the price or an advance payment. The "seriousness margin" differs from an "earnest money deposit" (arbitration); the earnest money deposit becomes part of the price if the purchase is completed, otherwise, it goes to the seller. However, the seriousness margin is intended to compensate the actual damage incurred by the agent (seller) if the principal (buyer) reneges on the purchase, and the remainder is returned to the principal. If it is insufficient to cover the damage, the agent has the right to demand the rest.