Is a merchant who trades with money he knows to be from bribery and usury sinful?

If a partner's wealth is mixed, containing both lawful and unlawful elements, then it is permissible to deal with him through buying, selling, and partnership, though it is disliked (makrooh). However, if it is known that the specific wealth he intends to trade with is entirely from unlawful sources, then it is not permissible to partner with him or work with him. It is preferable to advise the partner to repent and dispose of the unlawful wealth.