Yes Dallas can very well trade him, and may even do so. I think this may play out like Mack did last year. Fact is, if he hit the open market this year he'd be the top FA available. He's an excellent pass rusher that can also defend the run. There's been a lot of stats floating around stating how his production is at or near Mack and Watt levels over the past 3 years.
The open market is a crazy thing. I'm sure Oakland never thought Mack would find someone to pay his asking price and then the Bears swooped in. Obviously, I'm not saying Lawrence is better, but he'll find his asking price if he becomes available to all other teams.