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 season while shooting 39.5 percent from 3-point range on 3.7 attempts per game, gives Dallas a young, versatile 3-and-D forward who should be a perfect complement to the Mavericks' star-studded backcourt of Luka Doncic and Kyrie Irving.

And, by acquiring Williams in a sign-and-trade, Dallas still has access to its full mid-level exception, giving it an option to add another significant piece in free agency. Sources have told ESPN's Tim MacMahon that Dallas has interest in Portland Trail Blazers restricted free agent forward Matisse Thybulle.

Williams, the 22nd pick in the 2019 NBA draft out of the University of Tennessee, has been a key part of Boston's runs to the NBA Finals and Eastern Conference finals each of the past two seasons. While he vacillated in and out of coach Joe Mazzulla's rotation at times this season, he returned to a big role off the bench as this year's playoffs went on, just as he did a year ago under Ime Udoka as Boston went to the NBA Finals for the first time since 2010.

After getting significantly more expensive this offseason by acquiring Kristaps Porzingis from the Washington Wizards last month in a deal that saw franchise stalwart Marcus Smart wind up in Memphis, Boston opted to get two more second round picks rather than matching a deal to retain Williams as part of its roster next season, when the Celtics should once again be one of the top contenders in the East.

Williams, with his ability to guard above his 6-foot-6 frame and knock down open 3-point shots, was one of the more intriguing restricted free agents on the market this summer, particularly for contending teams. He's a career 43% shooter on corner 3-pointers, compared to 34% on all other 3-point attempts, per ESPN's Stats and Information Group, and is a versatile defensive player who guarded a wide variety of opposing players.

The Mavericks will now be hoping he can fulfill a similar role as Dallas hopes to bounce back from last season's extremely disappointing 11th place finish in the West, which saw the Mavericks fail to even qualify for the play-in tournament.

Williams had surgery on his left (non-shooting) hand after the season, but the Celtics said he was expected to be ready in time for training camp.

It's been a busy offseason for the Mavericks, who re-signed Irving to a 3-year deal, made two draft night trades to wind up with a pair of first round picks - center Dereck Lively and forward Olivier Maxence-Prosper - and signed guard Seth Curry.

San Antonio, meanwhile, gets itself to the salary floor by acquiring Bullock, a career 38 percent 3-point shooter, as well as landing yet another potentially significant future draft asset in the first round swap.