So wait, Prince was playing for a contender but wasn't playing well enough that they wanted to keep him so the contender traded him. Now he want to be bought out so he can pick another contender to try and play for. I think players should expect to honor their contracts. Detroit traded for him and wants him to help them make the playoffs. If I was them, I would not buy him out, I would expect him to play.
As for the Celtics, the only agreement that I can imagine was discussed was something along the lines of "if we can't trade you, we will buy you out". Now it sounds like Prince (or at least his agent) doesn't like who he was traded to. Prince played and played well enough to get himself traded. That was probably what he wanted. Danny did what he probably said he would do.
I see nothing here that could be put on the Celtics, this is now between Detroit and Prince, and Prince has no reason to be mad at anyone I don't think.