Is the sum of 500 pounds, taken from the family of the girl who engaged in reviling, slandering, and defaming, lawful or unlawful, after the plaintiff tore up the signed document he had in his possession?

It is not permissible to take financial compensation for emotional distress related to one's honor, because reconciliation for the hadd punishment of slander (qadhf) is invalid. This right is not financial, and it is not permissible to accept money in exchange for it, as it was legislated to safeguard one's honor.

Therefore, it is not permissible for you to take this money. We advise you to be patient and forgive the girl, seeking your reward from Allah.