I think the Celts may be able to appeal to Brewer's sense of pride. "You're the guy we need to shut down Wade and Kobe; we made a mistake letting Tony Allen go and you're that guy."
With Greg Popovich coming out and saying he doesn't expect any buyouts to contribute more than the players already on the roster, this could be a two horse race between Boston and Dallas, with Dallas the team that can offer him more money, and the Celts the team that can offer him more exposure (arguably).