Never talk to the bosses boss without going to your boss first. By doing so, you are ignoring the chain of command and likely will you fired. How would you feel if someone you managed went behind your back to talk to your boss?
If you really wanted to stay with that company because everything else is great, your only chance is to solve this with the boss of the boss.
5 figure bonus lost due to this is abuse.
I disagree that going above your boss first would lead to getting fired though (again with the exception of at will employment).