The NFL has gotten itself into trouble before when its constituent teams have been accused of colluding in certainly cases, whether it involved a specific player, or even a specific position group at any given time. Collusion in this sense is illegal. The league has been taken to court over cases and have settled for millions of dollars.
It is permissible, however, for players to ‘collude’. Collusion involves secret and unlawful conspiratorial communication between parties designed to exploit a situation to their own gain. Teams have been accused of communicating with one another before in order to suppress the market, to keep player salaries down across the board, rather than having a rogue team reset a market at a position with an outlier contract.