Logically, I can't see the Spurs being able to swing a deal to land Sheed. But just for sake of discussion here's an attempt at trying to inject some logic into a dream.
In order for this to work there needs to be a third team involved. That third team needs to meet the following criteria:
1. They need to be in a pure cash dumping mode. Be it for avoiding the luxury tax this year, trying to clear cap room for 2010 or whatever. Clearing cash is important, getting talent in return isn't.
2. They need to have a good young player on a reasonable contract that they could send to Detroit. Detroit won't move Sheed unless they get talent in return and the Spurs roster fodder won't cut it. The primary piece of return talent for Detroit would have to come from the third team.
3. They need to be a team who doesn't care about the Spurs or Pistons getting better. For example, New Orleans is facing some financial trouble and needs to dump some payroll, but they're not going to be a part of any deal that helped the Spurs out. So realistically this third team needs to be a borderline playoff team, or worse.
4. They're probably going to have an undesireable contract or two that the Spurs would need to agree to take back.
So the basic framework of the deal would look like this:
Detroit trades Sheed away in return for Player X.
Third team trades Player X for cap/tax relief.
Spurs pick up Sheed in return for helping Third team get cap/tax relief.
So is there such a 3rd team? Who knows. Even if there were what are the odds that both them and the Pistons feel the Spurs are the only ones who can help them out?