To Sir, Without Love: I'm Divorcing You, Vol. 1 (manga)

Daughter of a viscount, talented with a sword, and possessing a shrewd business sense, Byletta neither needs nor wants an arranged marriage. But on the day she’s supposed to meet her future groom…he’s gone off to war?! Why, a spouseless life with no man to keep her down is a dream come true! Skip to eight wonderful years later when she receives news that her stranger of a husband is set to return. Determined to keep her freedom, Byletta sends him a letter requesting a separation…only for him to respond with an outrageous wager: He’ll agree to a divorce—if they sleep together for a month…and she doesn’t conceive?! The audacity of this man!!