NFL Collective Bargaining Agreement
Salary Cap Accounting Rules
Section 8(b)Renegotiations and Extensions
(b)
No Player Contract, and no contract renegotiation or extension, may be agreed to between a Player and a Club for any term that expires prior to the last day of a League Year. All rights by a player to terminate a Player Contract must be exercised prior to the first day of any League Year to be terminated.