You call them Pactigons lower in the Ecology text; this should actually be in the monster title/name at the top if it's a racial name like Cornugon (Horned Devil).
The Devil's Deal ability has some issues- the text is confusing, and the ability is absolute such that even a deity can't break it- that's not kosher for a creature as low as CR 15. Deities are so powerful that they're beyond even the top-CR creatures in any Bestiary, except possibly for Demigods; saying that a creature as low on the totem pole as a mere CR 15 Devil could somehow get around the powers of a deity is laughable. I would change this to be something like "only a Miracle (with the material component) can break a Devil's Deal, once it is made, and the deity who grants the Miracle is likely to demand something akin to the pact the bargainer originally made with the Pactigon as compensation."
You also might want to give them some sort of "death curse" ability, activated upon their destruction; the fact that they're physically weaker than other Devils won't mean much to the average mortal who'd consider a bargain with one, but it would certainly mean they'd be bullied and attacked by stronger Devils in Hell itself. This would be mitigated by a death curse; other Devils would know it's a Bad Idea to attack a Pactigon, and accordingly let them be for the most part. Mortals who encounter one of these won't be so forgiving; most of them will see "Devil!" and swing/shoot first, ask questions later. Not necessary to the monster, just something to think about.
The stats are fairly well-crafted from what I could tell, though I admit I didn't examine them in careful detail yet. Certainly, a typical Pactigon caught in combat will do its best to escape rather than stay and fight, and this will make them good recurring villains for GMs who care to use them.
