If the Spurs decide to go under the cap, it will involve them waiving or even amnestying Bonner. If they go that route, they should have enough cap space to just take back Marion without having to send salary to Dallas, which I'm sure they'd prefer. The two teams could agree to that trade during the draft and finalize it after the moratorium in July. So the Spurs could pick their player, Dallas could get their cap space and everyone could be happy.
There's also the possibility of trading Bonner and non-salary assets to Dallas for Carter, Cunningham and 13. I'd actually like Carter a bit for than Marion, because the bench desperately needs a scoring wing. Carter could come in a play the three, and the Spurs could use 13, 27 or and the MLE to bring in another big and defensive wing.