Minkah Fitzpatrick was one of the league's best slot cornerbacks last year. Cowboys don't need a slot CB at this time. Fitzpatrick is too expensive (Dolphins want at least a 1st rounder). Trading Taco (2017 1st rounder) and a 2nd round pick is too much.
If you see a media/fan/blogger reference boundary or boundary CB with regards to the NFL, it means they're just throwing out terms they don't really understand.
Boundary CB is really a college football term. The hash marks are wide in college and the short side of the field vs the wide side are significantly different.
Boundary CB: Plays the short side and often has no Safety help.
Field CB: Plays the wide side and has Safety help.
The fact that many college QBs have limited arm strength also factors into the fact that a sideline route to the wide side is a longer and more difficult throw.
College QBs therefore prefer to throw to the short side which is why some teams keep their best CB on that side.
In the NFL teams either play their CBs right/left or have their top CB follow the top WR.