They are close to meeting the cap requirements--probably closer than giving up B,C or S.
How about Hayward and Horford agreeing to shave a little salary? They make 3-4X more than anybody else on the team. The disparity is pretty striking. That could be enough, or at least put them where they don't have to sacrifice any core team for next year.
Rozier would be the logical guy to go. He is expendable. Doesn't weaken the team to lose him.