Is it permissible to demand a debt after initially refusing to receive it, and then later being in need of it?

If the creditor waives the debt for the debtor, the debtor's liability is discharged, and the debtor cannot be demanded to repay the debt that was waived. The money paid by the debtor after the waiver does not rightfully belong to the creditor and must be returned to the debtor, unless the debtor himself permits the creditor to keep the money after being informed of the matter.