Reports here in NE claim that both sides were frustrated and that moving Devers was a win-win scenario.
Devers upset with the organization for bad communication and broken promises leading to loss of trust and the Bosox upset with Devers' team-second attitude, DH only position and large salary.
Context is important here too - Devers was signed to the massive contract to some extent to make good with the fan base for messing up with Betts and Bogaerts and losing them for largely nothing and show the league and other players they were willing to invest in the team again. I think Bosox ownership in particular signed him knowing it was an imperfect arrangement and with one eye toward moving him later.
Will the Bosox reinvest that money? Total unknown, but it does give them more flexibility and payroll to go after pitching which is what they really need. We'll see how Devers does in SF and whether they find him valuable enough to keep for the rest of his contract.