Haraguro Joushi ni Hamerarete Wana Kon

Keina witnesses her boyfriend's affair at his place. Six months into their relationship, and with marriage in mind, she is cheated on and dumped with a single message... Furthermore, when she tries to get back the money she saved for the wedding, she is scorned as a 'disgrace'. "You have no right to call me a disgrace! I will never cry myself to sleep over such a scum!" Swearing to get revenge, she agrees to a proposal from her handsome boss, Usui: "Wanna marry me?" Her goal is to hurt her ex-boyfriend's ego by marrying a superior-ranking man in front of him, but there's a high probability that he won't be convinced that she's marrying Usui, given how little she knows about him. They start by going out on a date to get to know each other better, but the more she learns about Usui, the more she feels a growing emotion towards him... (Source: MangaPlaza)