Part-Time Couple

Da-In's first love and former lover, Kang Seo-Jun, proposes a 100-day contract marriage to her, saying that if she doesn't get married, she'll die.
Although her pride is slightly hurt by the outrageous offer, Da-In boldly agrees to the terms of the contract, and thus begins her married life with Seo-Jun.

(Source: MangaUpdates)