Ibara Koushaku no Hanayome Kake wo Shimashou, Danna-sama

After her father dies in a political battle, Countess Giselle is on the verge of becoming homeless. But she is saved by Duke Bertram, who is rumored to be cruel and misanthropic. Thanks to a bet between her father and Bertram, Giselle becomes his wife...!? A love story with an age gap!

(Source: Shueisha, translated)