The seller may require security such as mortgage, lien or charge. A contract may also accelerate remaining installments after a missed installment. The buyer may undertake a charitable payment for deliberate default, but that amount cannot become the seller's income.
This rule is especially important in financing because cost-plus sale (مرابحة) is valid only when the financing remains a real sale. The sequence of ownership, possession, risk and final sale must therefore be visible in the transaction.