What is the ruling on taking a sum of money in exchange for cashing a large check, knowing that the check's issuer swears that the money's source is lawful?

The basic principle is the permissibility of dealing with lawful money. However, the owner's refusal to directly disburse it, combined with the offer of a very large commission, raises suspicion. The Prophet, peace and blessings be upon him, said: "Leave what makes you doubt for what does not make you doubt." If there is strong evidence that the money is unlawful, then it is not permissible to act as an agent for it or to assist its possessor in acquiring it.