They meet when they are in school. The fl has a strong character and has a blonde boyfriend. The ml was obsessed with the fl as she treated him with respect. They graduated and years later I think he buys her debt and makes her live with him. Eventually, the blonde boyfriend breaks off with the fl and that makes her extremely sad.
The relationship between the fl and ml looks very toxic, but hot nonetheless. For some reason, the fl takes a trip by sea, leaving behind the ml.

If I remember something else I'll add on.

Dont remember if it manhua or manwha

I remember reading something similar to what you described toward the beginning of the year.

Name: 'Til Debt Do Us Part / A Bittersweet Couple
Type: Manhwa
Description: Subin's mom routinely pays for matchmakers to set her up on dates. Subin turns them all down, one after another. One day she runs into an old friend from school, and things start to get complicated when he suddenly asks her to marry him. Is he the answer to her prayers or a devil in disguise? Will she take this chance to clear her debt?

Let me know if this is what you were looking for 😁😁

✔️ FL with red hair.
✔️ ML with black hair.
❔ He helped her clear the debt.
❌ I admit I don't remember there being a blond boyfriend.
❌ He is not the historical type.