What is the ruling on engaging in buying and selling with those whose lawful wealth is mixed with unlawful wealth, and is unlawful wealth forbidden in itself or because of its earner?

The ruling on dealing with someone whose lawful money is mixed with unlawful money, dealing with disbelievers in buying and selling, and accepting gifts from them, has been previously addressed. The hadith of Barirah indicates that if charity is received by someone who is permitted to take it, and then that person gifts it to another, it ceases to be charity, and it becomes permissible for someone who is forbidden to receive charity to partake of it. This does not apply to the issue of dealing with someone who possesses unlawful wealth.