Sources: Grant Williams heading to Mavs in sign-and-trade

The Dallas Mavericks and restricted free agent forward Grant Williams are finalizing a four-year, $53 million contract as part of a three-team sign-and-trade agreement with the Boston Celtics and San Antonio Spurs, sources told ESPN's Adrian Wojnarowski on Wednesday.

San Antonio will receive forward Reggie Bullock and an unprotected first-round pick swap in 2030 from Dallas, while both the Celtics -- Williams' former team -- and the Mavericks will receive two second-round picks, sources said.

The Celtics also will get a $6.2 million trade exception, per ESPN's Bobby Marks.

Acquiring Williams, a burly 24-year-old who averaged 8.1 points and 4.6 rebounds off the bench for the Celtics this past…