By restructuring or releasing Carr and Free, Dallas could create upwards of $15 million in cap space. And by extending Dez, he won't count all that much toward the cap. (That's why Dez has more leverage with the franchise tag than Dallas, because it would kill that cap space.)
Dallas can pretty much sign who they want. I trust Will McClay to evaluate bang for buck and put together a plan to VASTLY improve the defense next season.
Imagine last year with a prime pass rush, better free safety play, and a running back that can hit the home run.