Sale (and Bogaerts) signed after the Betts negotiations broke down. I got the impression they were Plan B.
If any single existing contract prevented the Sox from signing Betts, the only one that makes sense is Price with its $31mill AAV that went through the end of 2022.
Most likely it was the Big Picture with numerous 8 figure AAV deals including Price, Eovaldi, Martinez, Sandoval, Hanley and Rusney Castillo. Any others?