Wife Signed An Offer Letter When Accepting A Job, Now The Employer Is Stating That The Agreed Bonus % Was Incorrect And Is Halving It.

We are based in Florida. My wife started a new job in June of 2023. She and the VP of finance signed an offer letter which detailed her salary as well as an annual bonus of 12%. She recently earned a promotion which is pending not officially yet. In the conversation about the promotion her boss stated that her bonus % “will jump to 10%”. She then presented her original offer letter stating that she should be getting 12% annually. Her boss then stated that her pay band only qualifies for a 5% bonus and the offer letter is incorrect. Her boss then spoke to HR who confirmed this while stating that there is nothing they can do about it. Is it legal for them to essentially ignore the terms agreed to in the offer letter?

As info, my wife did receive a pro rated bonus for the second half of 2023 but it was very difficult to track what % she was actually given. We suspect that the payroll system had had her at 5% the whole time even though the offer letter has her at 12%.