Yeah maybe it's not so easy...
http://www.espn.com/blog/washington-...e-kirk-cousins
Franchise tag sign and trade: Because it would be a third franchise tag, according to the collective bargaining agreement it must be exclusive. That means only the Redskins can negotiate with Cousins. So, even during the legal tampering window before free agency, Cousins' agent would be unable to discuss contract numbers with other teams.
Yes, there are ways around that, but there are also ways to let other teams know this: Cousins won't agree to any wink-wink extension before free agency. Because of the restrictions, it would raise eyebrows in the league if there were a deal in the first minute of free agency that includes an extension.