I have been listening to all the Giannis/Brown chatter and I really don't think anyone out there has any idea of what is really going on or who is in the driver's seat or whatever.
What I think is interesting from a BOS perspective, is that they can play either side of a 3-way trade. If there is a team that is interested in Brown and willing to offer a package of X, Y, and Z, BOS could potentially just take X, Y, and Z instead of Giannis. A scenario I have in my mind is that there is at least one 3-way deal on the table but MIL is saying "OK, we will take X, Y, and Z but we also want BOS to add A and B".
This way, BOS can say to MIL, fine, have a nice season with Herro because we are just going to take the X, Y, Z deal over adding in A and B to get Giannis. Maybe BOS is offering Brown + C but MIL is trying to drive them up.