What is the ruling on the condition of a financial penalty for delay in an air conditioning installment purchase contract, and is the buyer sinful if he continues with the installments under the same contract?

A late payment penalty that benefits the seller directly is forbidden and constitutes clear usury (riba). However, if it is collected for charitable organizations, there is a difference of opinion among scholars. There is no sin upon the buyer who was unaware of the ruling concerning this penalty; the sin falls upon the seller. The buyer may continue with the contract as long as he is legally bound by it, and he must pay the installments on time to avoid the late payment penalty.