Gift for a Demon by Emily Alter

Dave never planned on becoming a human gift to the dethroned King of Hell.

Overlooked and cast away, Dave had to learn how to protect himself and fight his own battles. Of course, no one taught him how to survive in Hell, if that was even where he was.
Melchom had once ruled over Hell until a human betrayed and stole his crown. Now Hell’s paymaster and with the scars to show for it, he’s vowed not to make the same mistakes. But how could he say no to the most enticing man he’d ever seen? One who offered up his fear for Melchom to feast on so willingly?

It’s not an issue of chemistry. Hell is a complex ecosystem, and there are more forces and prophecies in place than either of them could’ve ever foreseen.