They can sue to get the money back, apparently. The precedent was set by Rickey Williams and Michael Vick. After those two cases, the court decided that “guaranteed money” is actually salary that is given up front, but is actually prorated over the length of the contract.
For example, if you get a 100 million in guaranteed money over 4 years (25 mil/year), but you get 100 mill of that in the first year, then quit, the team can sue you to get the 75 mil back. My interpretation is that it is equal each year but the court treats it like a book advance.
The difference here is that Haynesworth lied to their face, said he would play until the day they had to pay his bonus, then as soon as the check cleared, he went back to his previous stance.