By trade, you mean dump? If they trade and take back salary, they won't have that money to give to Mills. No matter what some people think of Danny, at $10M he is a cheap starter these days. If you get back another player making $10M, he probably isn't as good as Danny.
Like it or not, the Spurs are paying Tony's salary next year. There's on PG. Murray may be as good a distributor as Patty right now. He obviously lacks Patty's shooting ability - but then again, Patty can't hold a candle to Murray in penetrating and/or getting to the rim. On a per-36 minute basis, Murray scores just about as much as Patty. He just does it a much different way. And, of course, we don't know if he can maintain for longer minutes, or against starter-caliber defenders. And speaking of defense... can Murray be much worse than Patty on the defensive end?
I think your picture has the Spurs top two PG's for next year, and they will try and sign another older veteran to fill out the rest of the minutes. Maybe they can arrange a S&T with Patty, to get some value out of losing him. But I don't think they will be able to offer him what another team will. And they aren't known for guaranteeing starter minutes, which is something I think Patty wants in his career.