In the NFL contracts are only guarenteed for the season, this is why to see what a contract's really worth you have to look at the guarenteed money (or signing bonus). That's how that little part of the deal is worked around. This is why the length of the contract is never really the focal point. It's not like in baseball where if you give a guy 5 years 100 million, you're going to be paying him 20 million in that 4th and 5th year even if he turns into a Suppan.
In the NFL veterans are cut all the time to save cap room, or they're at least told to restructure their contract or take a pay cut with the threat of being cut if they don't.