Actually how the Franchise tag works is that the club tenders the player a contract as a Franchised Player with all the ramifications that that entails such as salary paid, guarantees involved and so on. The team then is given time to negotiate in good faith with the player to settle upon a longer term contract or the player can accept the tendered offer and be guaranteed the salary for one year regardless of whether the team cuts him or not.
Players who want to explore the free agent market usually despise this because they feel the club is impinging upon their rights to go out and sign a much longer term contract with major security whereever they might like to play. However, in many cases if a player wants to stay with his club it is perceived as just a fall back guarantee option until the inevitable longer term deal is completed.
It almost never happens that a player is offered the Franchise tender and then immediatelt accepts as the team, the player's agent and other teams can negotiate trades, trade compensation, longer term contracts and other contractual things.
So, it is possible for the Patriots to franchise Cassel and then negotiate with his agent until a deal could be reached that would satisfy both Matt's and the Patriots' needs. A 5 year $55 million contract that gives Matt a $15 million signing bonus, a very lucrative first year salary and then a structure that makes it palatable for them to trade Matt if Tom comes back healthy either next year or the year after is not only a possible scenario it is also the most likely considering the uncertainty of Brady's status.
Matt Cassel does not have to immediately accept the franchise tender. Sorry Crownsy but you are wrong there. Here is the NFLPA bylaws on the matter:
http://www.nflplayers.com/user/template.aspx?fmid=181&lmid=231&pid=540&type=cAs much it is unusual for this scenario to occur it is also a very unusual circumstance we are dealing with. First Cassel loves the Pats and if brady isn't going to be available would love to stay. But he understands that he isn't Tom Brady and if he comes back Matt won't be needed. The Pats also understand that Cassel has grown and deserves his time to start for a team. If Tom's not available then they want Matt. This is not the usual contentious negotiating that usually goes on with these designations. There is a clear understanding between the parties of what's best for all involved.
I say the Pats will franchise him, negotiate in good faith with him for a longer term deal and if Tom is showing great progress by draft day will make the trade for a pick and if Brady isn't then cassel will sign.