Accomplishing a trade may seem unlikely, or at the least, difficult, but that doesn't somehow mean the team wouldn't have tried anyway. After all, even if a team wasn't willing to give up a lot to make the trade, getting something for Dez would be better than getting nothing at all, so why wouldn't the team at least try?
Something I think fans tend to forget is that teams sometimes will make a trade even for a player they suspect might get released because in that way they are certain to get that player, whereas if the player is released there are 30 other teams that also may be in contention for that player. A team may not be willing to give up a lot for a player like that, but it's not unreasonable to think they might give up a late round draft pick, which again, for the Cowboys would be better than not getting anything.