The Spurs could really use Brown. They need his wing scoring and defense to really elevate into a contender this year.
There are several different trades that work financially but I'd prefer ones with Harper (instead of Castle) as the prime piece back.
This one works and makes sense for both teams
Brown, Scheierman
for
Harper, Johnson, Sochan, Bryant, Champagnie
Boston also sheds about 11 million before any additional moves to get to 15 players. Harper is the reason to do that trade, but Johnson should be a very nice wing option going forward, Sochan adds some size, and Bryant is a rookie lottery pick as well.
San Antonio loses depth and youth but has perhaps the best starting 5 in the sport with Fox, Castle, Brown, Barnes, Wemby with Vassell, KO, Kornet forming a strong top 8.
Next year if Boston brings back Sochan they'd be set up well to compete again if they can turn Simons into a decent cemter at the trade deadline. So 2026 starters - White, Harper, Johnson, Tatum, Trade with the bench being Pritchard, Gonzalez, Bryant, Sochan, Hauser, Rookie 1st, Queta, Minott, Garza, Williams (perhaps he is 2w again along with any 2nd rounders). If Tatum is close to what he was and Harper develops like you would expect that team could be nasty especially if Boston hits with what I'd expect to be a lottery pick next summer.