Is it permissible to accept money from an accountant for extra work, knowing that the source of his money is unlawful and I do not participate in his prohibited work?

It is permissible to work for an accountant and receive payment from him if the work itself is lawful, even if his money is mixed with unlawful earnings, as long as the payment given to you is not specifically from unlawful money. This is the preponderant opinion.

Some scholars have cited as evidence for the permissibility of dealing with someone whose money is mixed with lawful and unlawful earnings that the Prophet (peace be upon him) and the Companions dealt with the Jews, who engaged in usury.

However, if the majority of a person's money is unlawful, scholars have differed regarding the ruling on dealing with him. Some have deemed it permissible, while others considered it disliked or prohibited.

But if it is known that the money given to you is specifically unlawful, then it is not permissible to take it.