What is the ruling on trading with the money of a person whose wealth is mixed with usury – whether due to its presence in the حسبة (Hisbah) council or his dealings with people who have fallen into usury – if the trader takes a percentage of the net profit or loss, and he has advised the owner of the wealth and explained to him the seriousness of usury?

If you know that all of a person's money is unlawful (haram), then it is not permissible for you to engage in Mudarabah (profit-sharing) with their money. If some of their money is unlawful, then it is permissible for you to engage in Mudarabah with their money, though it is disliked (makrooh). The degree of dislike increases or decreases depending on the quantity of unlawful money.