Oshare-zuki Ouhi wa Gisou Kekkon de Ousama ni Dekiai Sareru

The manga's story begins when ordinary high school girl Chiyo Watanabe wakes up one day into a fantasy world, inhabiting the body of a Venessa, a lady of a ducal family. Though she thinks at first that she can enjoy the aristocratic high life, the discovery of her family's lordly sum of debts puts an end to that. Her last hope lies in her apparent betrothal to Alfred, the king of the Kingdom of Willand. Though the king does not seem to enamored with marriage, one year of a false relationship may buy Venessa the money she needs.

(Source: Anime News Network)