Free-agent guard Jason Terry has a verbal agreement on a three-year contract with the Boston Celtics, sources close to the situation confirmed Tuesday. Terry's offer from the Celtics is believed to be for the full mid-level exception of $5 million annually.

Sources familiar with the Dallas Mavericks' thinking told ESPNDallas.com that the team, coveting financial flexibility, is unlikely to match the Celtics' offer to Terry.

Terry won't be able to sign until July 11.

Terry spent the past eight seasons with Dallas, having replaced Steve Nash in 2004. He won the Sixth Man of the Year Award in 2009 and had a phenomenal playoff run in 2011, helping the Mavs win the franchise's only NBA championship.

A second source said that Terry's decision to move forward on a deal with Boston was made with no knowledge of Deron Williams' plans to either stay with the Brooklyn Nets or join the Mavs. On Tuesday night, Williams announced he'd stay with the Nets, agreeing to a five-year, $98 million deal, according to sources.

Terry, who will turn 35 before the start of next season, averaged 15.1 points and 3.6 assists last season and shot 37.8 percent from 3-point range. Fourth on the NBA's all-time list for 3-pointers made, Terry is a 38 percent 3-point shooter for his career and has averaged 16.1 points per game.