Shams Charania of Yahoo Sports reported the news Monday.
Matthews, 31, signed a four-year, $70 million contract with the Mavericks in 2015. The monetary value of his deal increased after DeAndre Jordan backed out of a verbal agreement with the franchise, according to Tim MacMahon of ESPN.com.
While the gesture was seen as an act of good faith by Mavericks owner Mark Cuban, there's likely a sense of buyer's remorse.
Matthews, who suffered a ruptured Achilles with the Portland Trail Blazers in 2014-15, has struggled to regain the form that led to his being one of the league's more underrated two-way players. He's shot just 39.5 percent in a Mavericks uniform, put up three straight points-per-game numbers that would have ranked as lows in Portland and not been as stellar on the defensive end.
Matthews also missed 19 games in 2017-18 because of injury. The Mavericks have also descended to the dregs of the NBA, tying the Atlanta Hawks for the third-worst record (24-58) this season.
"We never came apart," Matthews told reporters. "These last two years haven't been what anybody wants, but we stuck together and fought together. We never pointed fingers, and that's what I love about this organization."
There was no way Matthews was going to get $18.6 million guaranteed on the open market. He might have struggled to get that amount over a multiyear contract. There was never a chance he would opt out.