I think it's pretty straight forward. You're being asked to steal. You don't want to steal. So don't.

Sometimes standing up for what you believe to be right is a costly stance to take, or at the very least difficult.

If the worst case is that you get fired...well you were leaving anyway. What's to be lost? Even if for some bizarre reason they did withhold your salary, I'd personally rather than that be an accomplice to their wrong doing.