If someone forgives another for financial matters, is it permissible for him to retract his forgiveness?

If the holder of a right waives it, then the other party is absolved, and the former is not permitted to revert and claim it again, because what has fallen does not return. Scholars have agreed on the impermissibility of revoking an absolution, and the Messenger of Allah (peace be upon him) prohibited revoking a gift.