Bottom line is the Vikes are not going to just give Peterson away, they want to trade him for something. That is why all the talk is "we are not going to release him" is nothing more than the Vikes trying to drive the trade market and get a high pick from somebody.
The league is not dumb, they can see through this a mile away. They can say anything they want, but the bottom line is that no team in the NFL is going to pay a player $15 million when the player flat out says he wants to go elsewhere. We have experience with this, see Kyle Orton and Jay Ratliff, when players decide they don't want to play for you anymore, its almost impossible to "force them to play". Best case scenario is to just cut bait and move the player out because the last thing you want is disgruntled players in the lockerroom. So I guarantee Peterson is not going to be a Viking this year.
The only question is going to be which team pony ups the picks for Peterson? I could see a trade of maybe a 3rd this year and a 5th next year that could go higher based on Peterson's performance. Something like that might be doable. I really think it comes down to us and Arizona as the two teams in the running for Peterson. I would be willing to bet my posting privilege for a month that Peterson either ends up in Arizona or Dallas.