Cleveland says no, very quickly IMO.
They have a 23-year-old, that just averaged 21.6 points and just under 8 assists with 41% 3P shooting. He is locked up for the next 5 years at what will be considered pretty reasonable money starting at 33M. Why would Cleveland give him up for a player who, as good as JB is, is in the final year of his contract, with no guarantees (even if he verbally were to commit) that he will stay beyond this coming year. Even if Brown can agree to extension with them now, what incentive would he have? Sure, he is friends with Mitchell, but I'm sure he is friends with a lot of players. As we've seen from Dame's recent comments, there are markets/cities where players would welcome a trade to, i.e. Miami, LA, maybe NY. Boston may not be on the that list, but Cleveland is most certainly not on that list.
If Cleveland were willing to make that deal I would absolutely do that in a heartbeat. The contract that we have to sign JB to for him to stay has to be taken into consideration when you talk about the value of a player. Brown is certainly a better two-way player than Garland, but Garland is better offensively and will be making considerably less money, which could provide us with more flexibility to fill in the gaps going forward. Plus, Garland scares the heck out of me when we play the Cavs, which is the type player I want.