NFL contracts have guarantees for a reason, after those guarantees the contract can be terminated at any time. People assume that the length of a contract is a guarantee but I have yet to ever see one in the NFL, the money is the only guarantee...
So for player X to sign a 7 year contract, for $100M guaranteed - the only thing for sure thing is that player is getting $100M.
How and when that $100M is dispersed - via signing bonus, salaries, and other bonuses is entirely up to how those disbursements are negotiated.
So if that $100M guarantee is paid over the course of 3 years, the other 4 years are not guaranteed whatsoever and the team can decide if they want to continue or not based on the contractor's performance.