The Celtics can not have cap space until after the draft. Thus, the Celtics wouldn't be trading for the pick but the player selected by Dallas.
Additionally, the Celtics only have cap space if they remove all the roster holds. That means either sign the players to small enough contracts that you still have cap space or you have to let them go and lose Bird rights (they could still be signed as you could any free agent).
I guess what I'm saying is, these trades for draft picks using cap space are silly.