Mizuha ni Oshioki!

When the protagonist, a middle-aged man with no experience with girls, randomly asks girls out on the street, they immediately run away. This is seen by Mizuha, who laughs at him and gets him to treat her to an expensive dinner. There she straightforwardly seduces him into a game of 'he who cums pays up' which is inherently rigged in her favour. (Un)fortunately, things do not go according to Mizuha's expectations...