Is the amount reduced for prepayment of fuel purchased wholesale, or the extra amount paid for late payment, considered usury (riba)?

The ruling on this transaction (selling gasoline) varies depending on the state of the sold item. If specific, existing gasoline is sold, then delaying the payment is permissible (selling a described, absent item).

However, if gasoline described by its characteristics is sold (which is usually the case), it falls under the category of salam sale (forward sale), and delaying the payment is not permissible. Rather, it must be handed over at the time of the contract.

In the case of a valid sale, it is not permissible to oblige the buyer to pay more than the price if they delay payment, as this is usury (riba). Similarly, it is not permissible to reduce the price in exchange for prompt payment if it was by prior agreement.