The Demon's Bargain by Katee Robert

Lenora made a mistake. She trusted the wrong man, and in a moment of weakness, he stole a family heirloom from her. Her only chance to get it back will be during Samhain, when the Shadow Market opens and magical folk gather for three days to celebrate, and unfortunately she can't do it alone. She needs a demon.

Ramanu's entire existence is making bargains. They're a bargainer demon, after all. They've had their eye on Lenora for some time now, so they're pleased when she summons them to help with her little thief problem...especially because it means spending time in close proximity with her.

Lenora thought she could get away with a bargain that didn't require seven years of service, but when Ramanu informs her those are the terms, she decides to go it alone...even though the demon wants to tag along to the market. But as the clock ticks down and all Lenora's attempts to reclaim her amulet fail, making a deal with Ramanu might be the only option she has...

And they might take her heart right along with her soul.