Is it permissible to accept a job in the Islamic finance department of a global usurious bank, even though the salary will be paid by the usurious bank, and the employee intends through this to educate people about Islamic banking?

If the Islamic transactions branch within the bank operates according to the decisions of a reliable and trustworthy Sharia (definition) committee, and its work is separate from the interest-based operations of the bank, then there is no objection to working in it. The transformation of interest-based banks into Islamic ones is not impossible, because Islam is the origin of banking transactions.