The Cowboys have options. They do not have to rely solely upon Murray. We already know the Vikings are going to let Peterson go, and we also already know he wants to come here. If the Cowboys can get him for the same price range they are offering Murray they might let him go test Free Agency and the bigger contract.
I like Murray, especially given how he is playing now. Makes me giggle when I think of the people who were against us when we drafted him. I just feel RB is the easiest position to find a replacement for. Sometimes not as good, sometimes, better. I would prefer it be him, and I wouldn't be at all shocked if we franchised him so he can't leave. I've already heard rumors of a college RB that we like a lot.
The Cowboys do not need to break the bank. I don't blame Murray for rejecting the offer and wanting more, but if he wants too much, I'd either use the Franchise Tag or let him go, gain a compensatory pick for him, sign Peterson to a short term deal, and draft the kid we supposedly like.