As of right now, w/o trades or cuts, there are a MAX of four roster spots open next year...and that's assuming that neither Bonner or Vaughn are re-signed. Then you have to take into account the possibility of signing Scola, and the possiblity that Mahinmi will accept the Spurs offer this summer (a mandatory requirement of Spurs FO). If Bonner and Vaughn DO re-sign, that leaves the Spurs with no roster spots available and a necessity to reach a buyout agreement or trade with one or more of the players on the current roster....as Bruno eluded to earlier, that's easier said than done.
Your su ions/assumptions about Butler probably aren't right....remember, they were offered something for him at the trade deadline by the 76ers and didn't bite. He's had a rough start for sure, but I doubt the Spurs have given up on him.