Otsubone Reijou to Shuka no Kisetsu: Reitetsu Saishou-sama to no Jimu-teki na Konin Keiyaku ni, Fuman wa Gozaimasen

Alellia, who has had her engagement broken off, is past the marriageable age, and is known behind her back as the "domineering female supervisor". She spends her days performing her duties as the right hand woman of Marquess Istilia Wegmund, "the cold-hearted prime minister." One day, while performing her duties, the Prime Minister asks her to marry him. Rumor has it that the Prime Minister is in love with another woman. Alellia accepts the engagement, believing that she is being asked to enter into a contractual marriage. A newly-married life with the cold-hearted prime minister begins...! Source: MU