Per ESPN's Jeff Passan, the 2018 American League MVP will earn $27 million in 2020.
The Red Sox entered this offseason in a state of uncertainty with Chaim Bloom taking over as team president after Dave Dombrowski was fired in September and Brian O'Halloran being hired as general manager.
Team owner John Henry told reporters in September they needed to cut payroll to get under the luxury-tax threshold.
"This year we need to be under the CBT and that was something we've known for more than a year now," he said. "If you don't reset there are penalties so we've known for some time now we needed to reset as other clubs have done."
Betts and J.D. Martinez were the two most obvious trade candidates since they are the Red Sox two most expensive position players and could net a strong return given their consistent year to year performances.
It's still possible for the Red Sox to move Betts before the start of the season. MLB.com's Jon Paul Morosi reported on Jan. 2 the Los Angeles Dodgers have had discussions with Boston about the four-time All-Star.
At the very least, Betts got his deal for 2020 done and can get ready for spring training next month. The 27-year-old hit .295/.391/.524 with 29 homers and 16 stolen bases in 150 games last season.