You Call That Service?, Vol. 4 (light novel) — Kisetsu Morita

A date with...Sasara?! When Sasara suddenly finds herself in marriage talks with another noble, the only way out is to find a different potential mate -- and Ryouta, who can't stand seeing someone in need, volunteers for the role. Unfortunately, this means he'll have to defeat a swordsman in a duel and trick Sasara's parents into believing he and and their daughter are really dating-and neither looks likely. Will Ryouta have to get engaged just to help his friend?!