Shiro to Kuro - Youja to Migawari no Tsugai

The Uwabami, a serpent yokai, serves only one woman in his lifetime, granting her immense power in exchange for his essence. Hakuro, an Uwabami who lost his former master in the past, now has a new mate—Aguni, a man who was given his former master's heart and transformed into a half-yokai.

Having inherited the powers of a female yokai, Aguni’s body was altered—now possessing a female organ designed solely for mating with Hakuro.

The two make a living as handymen, constantly working together. However, Aguni secretly harbors complicated feelings toward Hakuro, who still cannot forget his former master…

Source: KiR comics (Google translate)