Why do jurists permit dealing with someone whose money is mixed lawful and unlawful, but not with someone whose entire money is unlawful, even though the prohibition relates to the person's responsibility and not to the money itself?

Unlawful wealth is of two types: 1. Unlawful in its essence, such as stolen or usurped property. It is not permissible to deal with the one who possesses it, as dealing with it constitutes an aggression against its rightful owner. 2. Unlawful due to its acquisition, such as wealth obtained through forbidden contracts like usury (riba) or gambling. It is permissible to deal with the one who possesses it through lawful means, because the unlawfulness relates to the earner's liability, not to the money itself.

Therefore, whoever deals with a usurper or a thief regarding usurped money, while knowing its origin, becomes like them in sin and liability. Ibn Rushd said, "Whoever does any of that while knowing, his path is like that of the usurper in all his affairs." Shaykh al-Islam Ibn Taymiyyah said, "If you know that he stole money, or betrayed a trust, or usurped it... it is not permissible for me to take it from him." However, if the situation of the money is unknown, the general principle is that whatever is in the possession of a Muslim belongs to him.