What is the ruling on borrowing money from a drug dealer for the purpose of lawful trade?

If a person sells something forbidden, or takes money in exchange for a forbidden act, then the money he has taken is unlawful (haram), and it is forbidden for him to benefit from it, because when Allah forbids something, He also forbids its price. If the money is unlawful for the one who acquired it, it is also unlawful for anyone who deals with him, because dealing with it is considered an endorsement of the unlawful. Therefore, it is not permissible to borrow from this man if the source of all his wealth is what was mentioned in the question.