Is it obligatory to return the money to the girl who gave it as a gift for the sake of Allah, and then demanded it back after a dispute?

If a woman willingly, without coercion, waives a loan, she does not have the right to reclaim it, because what has been relinquished does not return. However, if the payment was made in exchange for an illicit relationship, then it is unlawful gain (suht) and must be disposed of by spending it on charitable causes.