Why not just use sponsored debt?
Need a free fate point? No problem, here it is.
Want some Hellfire? Got some of that!
Boy, supernatural speed would be great right now. Good thing I got that!
Oooh that looks like it's gonna hurt! No prob, I'll suck it up!
Total debt = 4.
That's 4 compels the demon can do on the player.
It might not sound like much, but every time he's compelled into a situation that is not true to his own nature, it could affect his aspects.
He gets compelled to blast a mortal with Hellfire = Lawbreaker
There are probably plenty of "events" that would cause him to change an aspect which can be done at various milestones.
Anytime he's forced to take an Extreme consequence or taken out or forced to conceed, you can say that "the coin protected you from death" or something to that effect. Now he changes his High Concept...
I think a denarian coin is a slow process and should be done via role playing.
If your character wants to throw it away, let him and maybe he never thinks about it again. That's fine. Maybe that's all the role-playing you need to justify that he got rid of it...just remind him, every now and then, that he can always summon it back to his hand if he needs it...