Certainly, but as happened with Jon Lester and Andrew Miller- the "trade 'em , and get them back in free agency" maneuver doesn't seem to work out very well.And the fact of the matter is this: if they do trade Mookie, and he is 100% committed to going to free agency, and such a trade helps them get under the luxury tax (thus resetting it), they could, if they wanted to, shell out max dollars for Mookie after 2020. They'll have some big money coming off the books in the next couple of years so it is not an absolutely crazy thought. If they trade him and tell him they want him back and trading him now allows them to pay him what he wants as a free agent, and he still has a sense that being here is a good thing for him, it could possibly work.
In other words, trading him doesn't *necessarily* take them out of the Mookie 2020 FA sweepstakes. In fact, it might actually strengthen their position to sign him.