Jason Terry reached a verbal agreement today on a three-year, $15 million free agent contract with the Boston Celtics, a source close to the situation said.But just because Terry has reached that deal with the Celts, that doesn't officially end his eight-year run with the Mavs. Terry has said all along that he would allow the Mavs to match any offer he receives -- and now owner Mark Cuban is on the clock."He's going to give Cuban a chance to match it,'' a source close to the situation said. "We'll see what happens."I don't know why they won't (match it).''One of the most popular Mavs players in franchise history, Terry averaged 15.1 points and shot 43 percent from the field this past season for Dallas. He also was the starting point guard when the Mavs advanced to the 2006 NBA Finals, and was a key player off the bench when the Mavs defeated the Miami Heat for the 2011 NBA championship. The Mavs are currently in the hunt for Brooklyn Nets free agent point guard Deron Williams. So if they decide to match the Celtics' offer for Terry -- it's unlikely that they will match the offer -- it won't happen until Williams decides whether he's going to sign with the Nets or the Mavs."If the Mavs don't match it, you know where we're going,'' the source said. "We're going to Boston."We're going to be a Celtic. We're going to have to pull out some green.''Terry is currently in Orlando with his girls' youth basketball team at the AAU Nationals.