When you franchise a player, you have to have sufficient cap space when you make the designation, not when the player signs the franchise tender. And you have to keep that amount of cap space available unless you withdraw the tender. So the Patriots would have to free up $14.6 million in cap space before the free agent period begins. And my guess is that Cassel would sign that tender before the ink was dry. That would effectively put them out of the free agency business, and might cost them the chance to retain their own RFAs or UFAs.
Cassel has worked hard for the past four years, and is justifying the confidence that Belichick has in him. He deserves a chance to sign a contract that provides lifetime security and gives him a chance to be a starter. Let him do well here this year, wish him well, work with O'Connell and develop another QB next year. It's the Patriots way.