This is just another personnel situation the Cowboys mishandled. I think developing him and then deciding to trade him away would be smart. But trading for him, never and never letting him take the field has not increased his trade value at all. In fact, he is another year from the draft so my guess is they will get offered less than what they gave up for him.
The should have given him some playing time in blowout games, even if they had kept him active for some games. Let him get some experience. If he has one game where he looked good, he trade value would increase significantly.
If they re-sign Dak, there is no point in keep Fields. I think they will eventually trade him away for less than what they got, or they will lose him to free agency and get nothing. This is how the Jones Boys roll.