Circumstances of a (Temporary) Bride

(Kari) Hanayome no Yangotonaki Jijou: Rikon Dekitara Ikkaku Senkin!
(仮)花嫁のやんごとなき事情 ~離婚できたら一攫千金!~

Adaptation

novel

Ferdia is a workaholic orphan living in a church orphanage, helping take care of the younger children. One day, the King asks her to take a job substituting for his sickly younger sister. No problem—it's not her first time doing it, and the pay's gonna be great. But this time, her job is to wed a prince from an enemy country... And make him divorce her ASAP! (Source: J-Novel Club Forums)