(1) Dallas agreed to take on Stevenson's bad contract
(2) Washington can still get a salary dump for Jamison elsewhere
(3) It appears that Boston never agreed to any deal. Merely that they were interested. And I doubt it ever got to a point were they were willing to pull the trigger due to the financial consequences.
(4) Washington still isn't sure if they want to trade Jamison. They realize it would help fast-track their rebuilding process but they also value Jamison as their leader and as the player who turned around the franchise several years ago. They like him as a veteran influence on the younger guys. Being able to get that cap flexibility while keeping the option open to retain Jamison was a plus for Washington.
I think Washington would be extremely foolish to hold onto Jamison. They aren't going anywhere with a Jamison/Arenas combo and the 2 make too much money to make significant FA additions, Arenas is untradeable right now, and gambling on Arenas contract getting voided through a felony conviction or league ruling is too risky.
And it's not as if they have a stable of young studs that will thrive if Jamison stays on board as a 14-million-dollar mentor. Nick Young and Randy Foye are mediocre shooter/scorers who hate to pass, rebound or defend, Andray Blatche hasn't exactly taken to veteran leadership in the past and Javale McGee, while an interesting prospect, is hardly a crucial building block.
They need to free up cap space to target some mid-level young players without feeling the pressure to hit home runs and tank to improve their draft position.