Is a Muslim sinful for writing or sending letters via fax to insurance companies or banks if he suspects the unlawfulness of the content of these transactions due to his work?

If you know or strongly suspect that the letters contain prohibited matters, such as commercial insurance or usurious transactions, it is not permissible for you to write or send them. This is because doing so constitutes assisting in committing a forbidden act, based on the generality of Allah's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Regarding usury, it was narrated from Jabir, may Allah be pleased with him, that he said: "The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes usury, its payer, its scribe, and its two witnesses, and he said: 'They are all alike.'"