Gakeppuchi Reijou wa Kurokishi-sama wo Horesasetai!

Her family is in debt, and they don't have enough money for her sick sister's treatment...
The troubled noblewoman, Clarice, finds herself engaged in an arranged marriage with the black-knight Frost, the most fearsome warrior of the kingdom, known for "slaughtering beasts with his bare hands"! In an attempt to secure her fortune, Clarice tries to seduce "Sir Black Knight," and that was when she discovers "her" secret…
(Source: Yuri-Hime, translated)