There is no rule that prohibits a team owner from talking to a free agent or player on his team. They can discuss player compensation. There is a rule that only the players’ union certified agent can agree to terms. Sans the agent agreeing to the terms the contract cannot be approved by the NFL or NFLPA. Players can represent themselves, but must formally notify the league and Players’ Association of their intention to do so.
Obviously the NFLPA frowns on it. Few represent themselves. Note, the rule applies to players that are not members of the union. There’s not many of those as all players, whether they join the union or not, must pay union dues.