If the Spurs are intending to take Sexton into cap space, then the rules aren't any different. In fact, RFAs don't have unique S&T rules at all, but players who are RFAs often run into BYC consideration because they tend to receive contracts in excess of 120 percent of their previous salary. Sexton (or Lavine for that matter) would trigger those conditions if the Spurs are trying to acquire him as an over-the-cap team. Cleveland would only be able to take back about 60 percent of the salary they send out, but the Spurs would have to account for 100 percent of Sexton's salary. This leaves a pretty small number of ways to have both sides fit into trade rules.
Cleveland is near the tax though, so the Spurs are probably not going to get a deal where they keep their cap space. So unless they get a good deal from Cleveland where they can push Sexton's APY down substantially, it might be better for them to just price the Cavs out.