There is no need to write an IOU. The job of the cashier is to manage the money. The cashier collects the reserve funds for the company, not for his own use. The cashier is just an executor. If the boss can't even trust this thing, he can't hire people to do things. In addition, it's obvious by looking at the detailed accounts and bank statements that he really doesn't trust the boss.

If that doesn't work, you can make a registration form. When filling out the check, register the bill number, amount, person handling it, etc., and have the boss sign it. This is also a method