Is the money received by an auditor lawful, knowing that he audits various companies, including banks?

Calculating and reviewing usury is akin to writing it and bearing witness to it. The Prophet, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses, considering them all equal. This prohibits writing the transaction between those dealing in usury and bearing witness for them. It also prohibits assisting in falsehood. The money taken for this work is ill-gotten and forbidden, because it is wages for a service that is religiously prohibited.