There's a subtext that you guys are missing. The devil wins whether he says Johnny won or lost.
If the devil wins, he gets Johnny's soul per the terms of the bet.
If Johnny wins, the devil gets Johnny's soul because he's committing multiple deadly sins purely by boasting and challenging the devil for a golden fiddle: pride, greed, vanity.
If he deems Johnny the winner, Johnny is less likely to challenge the decision, and he's also more likely to continue committing the sins of pride and vanity (by boasting of his victory over the Devil Himself).
Johnny's only chance to avoid his fate (per most strains of the Christian faith) is to acknowledge and repent/confess/absolve his sins by seeking divine mercy. The devil knows this is unlikely.
Except everyone is a sinner by default, so what's the downside? That's the problem with these "temptation" parables. Nothing Johnny could do himself would ever avoid that fate, so why act like it?
I'm just pointing out that within the constructs of the predominant Christian mythologies... Johnny's fucked himself over unless he recognizes he's being a shit and atones for it.
Whether you assign any validity to the underlying dogma is an individual choice. People have a tendency to overlook logical inconsistencies in their faith frameworks, in any case.