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.