According to ESPN Insider Kevin Pelton, Mavs can make the best offer for Cousins.
His suggested proposal from the Mavs looks like this:
Mavs: Darren Collison, Jae Crowder and Brandan Wright
The framework of a possible deal with Dallas would include Wright as a replacement for Cousins, a choice between guards Collison and Rodrigue Beaubois and one of the Mavericks' rookie wings (Crowder or Jared Cunningham). On paper, this version comes out best of all trade options, though there's a significant catch: Either Beaubois or Collison would become a restricted free agent this summer, while Wright is unrestricted. So there's no guarantee Sacramento would get anyone besides Crowder for the long term, and both Collison and Wright are likely to command raises on the open market. This deal also isn't a particularly good match for the Kings' needs. Crowder, a favorite of statistical analysts in last June's draft, could help out at small forward if he improves his outside shooting, but Collison essentially duplicates what Sacramento already has at point guard, and Wright isn't the defensive anchor the Kings require in the middle.
The Mavs don't have the assets to make an offer that makes perfect basketball sense for the Kings. That's why the Mavs would have to make it about dollars, taking back a bad contract or two.