Why Boston (might) do this:

Donny has 3 years left on his deal and with JB due for a massive extension it gives Boston an opportunity to have another all NBA player next to Tatum for the next 3 years. Boston adds a first to compensate for the years of team control.

Why Cleveland (might) do this:

After flaming out in the playoffs, it was pretty clear that it is going to be difficult to fully optimize both Garland and Mitchell. Garland and Brown complement each other more and Cleveland could potentially be in a better situation financially to extend Brown.