My view of the timeline, anyone can add or fix anything, I'm sure most of this isn't news for most, but just doing a typed up time line.
It is a solid mix of rumors/reports/hearsay.
Off-season 2020-2021, Hall was a free agent.
Rumors Bruins in on Hall.
Article released that Hall was willing to sign with a contender for less money.
More rumors that Bruins were in on him and it was close.
Speculation (possibly with some verification?), that paraphrasing, Bruins asked Hall if he could wait for them to clear some cap space so they could make it work.
Shortly after, it's announced the Hall is signing with the Buffalo Sabres for 8 Million dollars. Eyebrows fully raised by many, but Buffalo optimistic.
Don Sweeney had done nothing but add Craig Smith on forward.
February to April, Bruins had a strong start, followed by a fairly mediocre run filled with half the roster injured and goal scoring problems. Buffalo had record setting losing streak and Hall scored less goals than 410 players in the league as of mid-March.
Hall 100% being traded, removed from line up to avoid injury.
Rumors that Boston was in on Hall and the offer was Bjork and 2nd. Many thought Hall would get more, but analysts think he wouldn't get a first.
Hall (and Lazar) traded for Bjork and a second.
Edit: That about sums it up.
Additional note: Me eating crow that the Bruins shouldn't get Hall. Definitely hopes it works out for him and us.