Cause I’ve told you once, you son of a bitch, I’m the best there’s ever been

Some people have an interpretation that the devil wins anyway, either because of the sin of pride or because Johnny is gambling with his immortal soul.

That's not in keeping with hopeposting though, so let me offer an alternative interpretation.

The devil sees this good ol' boy playing fiddle in the woods and thinks he's an easy mark. He proposes the wager, and then in true devilish fashion immediately cheats.

Johnny's unbothered by this. He's just that good, and he schools the devil through skill alone.

Johnny's a force for wholesomeness. He's just happy to be sawing away at the fiddle in the woods, singing songs about chickens and petting dogs. He's talented, but he's also incorruptible.

Because of this, the devil can't swindle him. He truly is a good ol' boy. And when you think about it, wager or not, there's nothing damnable about playing good tunes.

Usually in these Faustian tales, the devil offers a wicked person a bargain, and then that person uses the bargain to perform more wickedness. I posit that the devil doesn't win souls just because of the contract, but because the person damns themselves through their actions after the deal is struck.

If Johnny was a wicked person, maybe he'd waste the golden fiddle on hookers and blow and end up damned anyway. But Johnny doesn't seem like that guy. He probably keeps the gold fiddle on the mantle to tell the tale of his jam session. There's nothing wicked about that, so Johnny is safe

He probably keeps the gold fiddle on the mantle to tell the tale of his jam session.
- Assuming a typical fiddle weighs 400g and was made from wood with density of 0.8g/ml, the same fiddle made from gold would weigh~9.5kg though it could be lighter as the walls could be made thinner if made from gold sheet i would imagine
- It's Appalachia. His mantelpiece is probably one massive tree trunk.
I like your alternative interpretation

I though always thought (for some reason) that Johnny didn't accept the golden fiddle. And I checked the original lyrics again, there's nothing about him taking/not taking the fiddle, but I always imagined (when listening to the song) that during "I told you once, you son of a bitch, I'm the best that's ever been" Johnny just fucking kicks the golden fiddle in the direction of the devil - making him run away with even more shame :)

Dang, this is like a triple espresso shot of hope. I can see the sounds of it, AHHHHHHH

That was Asmodeus' he didn't tune his cello before he left hell!