Just some factual information in response to you voicing your concern over the cap. Mathis has a salary of 2.4m this season. His bonus money only counts against his original team if he gets traded. So the Pats would only have to pay him his remaining salary over the next 10 games. 10/16 x 2.4m = 1.5m left in paychecks in 2011.
Robert Mathis Contract, Salaries, and Transactions
The Pats are very capable of absorbing Mathis' cap number for this year. As far as resigning. That question would only come up later IF he performed very, very well.
Thanks for the update. Even with those numbers I'd still pass. But that's just me.