How is the remainder of the installments calculated if the buyer wishes to pay them before the due date, and is the installment considered unlawful in certain cases?

The agreed-upon price when selling an item is binding on the buyer and is a right of the seller. If the seller wishes to forgo some of his right as an act of kindness, there is no objection. There is also no objection to the buyer paying the installments earlier than their pre-agreed due dates, but this should not be in exchange for a reduction in the installments.