Is the statutory surplus ruled in favor of the creditor – in excess of the principal debt – lawful or unlawful money?

It is not permissible to impose a late fee on a debtor for delaying the payment of a debt, because it is usury (riba), which is forbidden. This is the usury of the Ignorance (Jahiliyyah) that the Quran prohibited, and it obligates granting respite to the one in hardship.

However, if the debtor is procrastinating, it is permissible to sue him to compel him to pay the debt. It is also permissible to charge him for the litigation fees, lawyer's fees, and expenses incurred by the creditor due to his procrastination.