The World Federation One Stop Fiqh

Ruling 2175

If a person is aware of the amount he owes but his creditor is not aware of it, then in the event that the creditor settles the debt for an amount that is less than the actual amount – for example, he is owed £500 and settles the debt for £100 – the extra amount [i.e. £400 in this example] is not lawful (ḥalāl) for the debtor unless he informs the creditor of the actual amount he owes him and seeks his consent. Alternatively, the situation must be such that had the creditor known the actual amount of the debt, he would still have settled for the same [lesser] amount.