I don't think he gets bought out at all. I also see no reason why he would come to Boston.
Wouldn't him playing a prominent role on a top seed in the east that could surely use his skill set be pretty appealing to a guy about to enter free-agency?
As for him not being bought out, if they can get a second round pick or something for him, obviously they do that, but if not, what would be the point of keeping him? He arguably could help them win a game or two which would hurt their lottery pick. He is 9 years older than garland and sexton. Why would ownership not jump at chance to save a little money? Plus they already are trying to deal with Love and his albatross contract in the frontcourt.
Thompson is still just 28. The Cavs love him and want to re-sign him as none of their young guys (other than Nance) are big guys. He also hasn't asked to be traded, asked to be bought out, etc. Now maybe that changes, but by most accounts Thompson actually likes Cleveland and likes playing for the Cavs. I just don't see him getting bought out as all that likely. Now a trade could certainly happen, especially as I think a lot of teams will be interested in a solid defender that rebounds and hustles and whose contract isn't terrible (18.5 million this year, so doable for most teams to acquire).
He is 29 in two months. Sexton turns 21 tomorrow. Garland turns 20 in a few weeks. You really think it makes sense for them to resign Thompson when he is 8-9 years older than their franchise cornerstones? You think Thompson wants to spend the last few years of his prime playing on 25 win teams with two raw young guards? Literally none of that makes any sense. The cavs can hopefully get a one player or pick for Love and hopefully can get a frontcourt player with their lottery pick this year. Also, how is 18.5 doable for most teams? Which contenders have assets to trade for him?
Thompson is the exact type of player young teams love to have. He isn't going to hurt their draft position, but plays hard, plays the right way, is well liked, etc. There is absolutely no reason in the world, the Cavs wouldn't bring him back. Now sure, maybe he wants to play for a contender down the line, but the Cavs can, and probably will, offer him more money than anyone else can reasonably offer. And since Cleveland isn't going to be a real player in free agency for a few years, they won't hurt themselves by keeping on a solid veteran and citizen that is well liked and won't take playing time from their franchise cornerstones.
Dallas, Miami, Toronto, Denver, Indiana, LAC can all easily acquire him without harming their cores that much. Portland and Brooklyn could as well (not really top teams this year, but clearly not tanking teams either). I also don't think Cleveland would be opposed to taking on future salary so a team could potentially use a Thompson trade to clear future salary. Charlotte, for example, in a swap of Thompson for Zeller might make sense for both teams. Zeller is cheaper, but has the extra year. That is similar to what the Cavs did in the Clarkson for Exum swap. Several teams might fit that type of trade partner for Cleveland including the Wolves (Dieng), Knicks (several players), Kings (several players), etc.
If Thompson becomes available, I think his market will be pretty strong. And I don't mean strong enough to get a 1st, but strong enough to give the Cavs several trade options and enough to trade him where that makes far more sense then just buying him out.