30‑sai Toshiue Koushaku no Gosai no Hazu ga Sono Musuko ni Dekiaisareru

Serena is a poor aristocrat doing sewing work to help her indebted family. A marriage proposal comes from Marquis Harding, who is 30 years older, offering to take on all her family’s debts. She agrees for her family’s sake, and goes to Harding mansion — but instead of the Marquis, she meets his son Felix, admired by many women. It turns out her intended marriage is not with the Marquis himself, but with Felix. Serena expects Felix to be cold, but to her surprise, he begins to dote on her, showing intense affection.

(Source: Comico, translated)