Teito Oni Koi Monogatari Susu Kaburi Reijou no Kekkon

Harunei, a fallen noblewoman who lost her mother, takes refuge in a baron's household. She is treated like a servant, and her mother's keepsake, an amethyst hairpin, is sold. Harunei visits Keishin, the art dealer rumored to be a demon, who bought the hairpin. The condition for getting it back is a "political marriage." He wanted to use Harunei's family background for his business.

"I, with the eyes of a demon, exist only to manage cursed items," Keishin says, closing himself off from the world. But his eyes shine like the amethyst in the hairpin, enveloping Harunei in warmth. Now it is Harunei's turn to sincerely wish to save him, a man who has never been loved by anyone...

(Source: Kadokawa, translated)