Minnesota is not going to say anything about releasing him because that wouldn't be good business. If Peterson wants out, though, he's not going to be kept by the Vikings. They will release him in April because you don't pay that kind of salary to a 30-year-old back who doesn't want to be there.
In the meantime, they will field offers for him. The trouble is, as you said, not many are going to be standing in line to give up much for an old back. I mentioned in another thread offering Joseph Randle for him, and I meant it. Dallas doesn't need to give up draft picks or starters to get Peterson when he'll be free to acquire in April, but if it can get the right to negotiate a contract with him (before trading for him), it could end up getting a better deal than it would in bidding for his services (and a better one than it could negotiate with Murray). I don't mind even throwing in a conditional fifth-round pick for 2016, based on his play.
He's a couple of years older than Murray, but he's had a better career healthwise and productionwise.