Well, the trade doesn't make sense either way.
But the Spurs can play hardball with V-Span because if worst comes to worst, V-Span in training camp is something they can live with. And V-Span knows that. So that gives him zero leverage and it'll be why he can't ask for a buyout from the Spurs, like that Croatian got last year from the Hornets.
If V-Span was a guy the Spurs would never want on their team, he'd have a lot more leverage. He knows that there's a chance the Spurs would just keep him if he doesn't void his contract completely.