Is the agreement between the buyer and the seller to pay the price of the commodity after six months with an increase of two thousand pounds over its original price considered installment sale, or is it a form of circumventing usury?

If a man buys an item and then agrees with the seller to postpone payment in exchange for an increase in price, this is forbidden usury (riba). However, if he has not yet bought the item, and agrees with the seller to purchase it at a future price higher than its current cash price, this is permissible.