Yonoi Tsukihiko's Happy Hell Vol. 3 (manga)

To save their family farm, Mikan must marry Tsukihiko, the heir to a highly influential food and holdings company. They meet and find they can’t stand each other--but not standing each other doesn’t mean they can’t have a one-night stand. One night becomes two, and two becomes three.

Pretty soon, the cool and calculating Tsukihiko begins to develop feelings for Mikan. But having feelings is something new for Tsukihiko, so he sets conditions on their relationship: They're together, but not officially dating. Of course, Mikan has lots to say about that, and there will be hell to pay if Tsukihiko doesn’t listen.