In Dallas, they get two young back court players who are already friends and who can lead the next generation of Mavs. I am assuming that Mil can't bring Jennings back due to 'irreconcilable differences' and will be able to keep Collison on a reasonable deal.
For the Raps, they get a better fit in the back court with Gay and Lowry as Mayo is a better shooter than Derozan, and we get a backup pg in Roddy who knows Coach Casey's D system. And we get a hustle player in Crowder for some wing depth.
This also reduces our Cap hit next year as Derozan is set for a raise and Mayo will be making about half of what DD will be. Which gives us room to Trade Bargs for a player like Boozer who has a bad contract (but is still a very capable 4) or for a player like Millsap who will be getting a raise in the offseason or for someone else altogether (Big Al? J-Smooth? Pau?)