Everything the Devil says is a lie.
He isn't bound by some mystical rule set; the rules of his deals, as he explains them to Morgan, are entirely arbitrary and self-imposed. (This explains away a few contradictions in the series' mythos, such as whether or not the Devil has the power to override free will.) He doesn't make deals because he needs souls, and he doesn't keep Collectors around because he needs help collecting souls. There is no balancing bad luck, and Morgan only saves people from their bargains because the Devil decides to let them out. The Devil just enjoys sowing chaos in the world solely For the Evulz, and having people around him to manipulate makes things more fun.
The last 48 hours account for leap years.
The ambiguity over whether the deal is for a ten year period or ten standard 365 day years allows an in-between zone where the deal applies only partially, as any ten year period would have at least two days added to it in leap years.
The Devil is fulfilling his classic role as the Prosecutor.
"Satan" means "the adversary," and it's often used (especially in the story of Job, to whom Morgan bears no small resemblance) in the sense of a prosecuting attorney. And a prosecuting attorney is meaningless without their counterpart—a defense attorney, who presents the client's side of things, looks from the client's point of view, and makes sure that the client's rights haven't been violated. Morgan fulfills that function: he learns about the client's experiences, attempts to arrange reparations, and calls the devil on contract violations.
The devil even gave him a hint—the first client Morgan was given after he negotiated for the ability to redeem was an overzealous prosecuting attorney who dealt for the ability to win all his cases but forgot to ask that only the guilty be condemned. Similarly, Satan collects on all his deals, even those of people who made their deals out of ignorance or in order to do good—he takes the innocent along with the guilty. He's an overzealous prosecutor himself, and Morgan is his checking force.
Which raises the question...was the devil blessed/cursed just like Carter Baines was, to always win whether or not the party concerned is guilty? If so, what the hell was powerful enough to do that to him? And—without even knowing it—is Morgan working towards the devil's redemption? Carter Baines was redeemed by, among other things, releasing those whom he wrongfully convicted. Isn't that what Morgan does, in a sense? Is the devil trying, through Morgan, to reset his own clock and earn his own escape route?
