I don't think anyone would dispute that we could afford him. I'm anticipating around 35-40 mil in cap space for us after a few cuts and the cap figure gets sorted out. That's even after we resign Mack.
Now as to the idea of Dallas choosing Martin over Lawrence.... Their are only a few positions teams spend draft pick after draft pick trying to find "The guy" Edge Rusher happens to be one. Once you think you have one, you never let them go, unless you have to, or they have other issues you know about. (*Patriots do not apply they are nuts)
Either way, it's a whole lot easier finding a competent guard than it is finding an elite pass rusher. You have to resign one of the hardest positions to fill.