Yeah, it wouldn't make much sense. They are just under the cap I think with about 4.5 million left, they could sign Terry to that then do a sign and trade, but that would make Terry take a paycut. Dallas could possibly be looking for other parts from Boston and another third team so that they can get over the cap to get the full mid level.
No, Dallas still has Terry's Bird-Rights (contrary to reports, they have not renounced him yet). Dallas could sign him to any amount it likes, so long as the C's can send out at least 66.666% (the opposite of 150% of incoming) of Terry's salary. Ideally, the players Boston would send out would go to a 3rd team who has cap room to absorb such players.
As long as Dallas takes on no salary in such a trade, they would not lose a single dime under the salary cap. Dallas would also stand to gain a trade exception in the amount of Terrry's salary (which would not count against the cap).
To grease the wheels, we most likely would have to send picks to both Dallas and the 3rd team. Dallas might be wooed with just the trade exception though.
Basically, so long as we can find a 3rd team willing to take on enough salary, we could sign Terry to the same amount as the MLE without having any affect on Dallas' cap number.