What is the ruling on taking money from a person and then discovering that it is unlawful, and what should be done with it, especially if the owner of the money is a non-Muslim girl, and does the ruling differ if the money was taken as a form of assistance?

If the sacredness of property is due to the attachment of another's right to it, such as stolen property, then it is not permissible to accept or retain it, and it must be returned to its owner. However, if the sacredness is due to the method of acquisition, such as usurious funds, then if the owner possesses nothing else, all of it is unlawful, and it is not permissible to benefit from it or accept it as a gift. But if he possesses other lawful property, then his wealth is mixed, and it is disliked (makruh (definition)) but not unlawful (haram (definition)). Some scholars restrict the consequence of unlawful wealth solely to its acquirer.