I Can't Refuse S Vol. 1 (manga)

Miku is in a bind: her father has taken on too much debt, subjecting her family to so much work and stress that now her mother has fallen ill. Miku is willing to do what it takes to help her family, so she agrees to become a wealthy benefactor's mistress to raise the cash they need! Unfortunately, that alone isn't enough - the wealthy man has a preference for women who are experienced in bed, and Miku is anything but. That's when the man's butler, Kamishiro, steps in.

The gorgeous but sharp-tongued Kamishiro is blunt: he says he's been sent to train Miku in the sexual arts. Under his hot and heavy tutelage, can Miku become experienced enough to inflame the desire of any man... including her teacher?