But you don't though.
Trading him now accelerates his remaining prorated signing bonus and actually costs an additional $11.1M in cap room. Which means the Cowboys would have to somehow create $5.9M in cap room immediately just to be able to trade him.
If he was traded, cut, or he chose to retire this next off-season, it would free up an additional $5.1M in cap room (his 2017 cap hit of $24.7M cap hit minus accelerated prorated bonus hit of $19.6). It also means the Cowboys are no longer paying him a $14M base salary (real money, as opposed to fictional cap space) AND he is completely off the books when the 2018 season starts.
If he was cut or retired next off-season, and the team decided to make him a June 1st cut, the team would gain $14M in cap space, but that would not be available until June 1st, long after most free agents have signed. In that scenario, Romo would still be on the 2018 books for $8.9M as well.