Cowboys CEO and Director of Player Personnel, Stephen Jones, was a guest on Tiki and Tierney in part to kill the idea of Johnny Manziel to Dallas.  Jones says there's no chance that the Cowboys sign him.

Via Tiki and Tierney: “No, there’s no chance,” Jones said on Tiki and Tierney. “Obviously Johnny needs to get his hands around his life. He needs to do the things that make him a good person off the field, I think, before he can ever have any chance of being successful in our league. He’s a guy obviously from Texas that we’ve all followed and had an amazing career there at A&M. Unfortunately, he has some things that are holding him back not only as a football player, but as a person in life. We certainly hope he gets his hands around that because he’s obviously a guy that’s had some very unique successes in his life. When he is able to have his hands around some of his issues that he has off the field, then it usually translates into a success for him on the field. But I hope he is able to get his hands around his life, and at some point if he does, I’m sure he’ll get another chance. But at the moment, it certainly won’t be in Dallas.”

This has to be a clear sign that Manziel's NFL career is over.  They gave Greg Hardy another chance.  Hell Hardy would still be on the Cowboys if he wasn't such an asshole to his teammates.