This is a turning point in the story as it’s the point where the MC’s actions directly affect my enjoyment of the story. Simply put, if he doesn’t respond in a reasonable way that I can relate to I won’t be able to put myself in his shoes and the story loses much of its allure. If I had found out that she tricked me it would ruin any chance of a relationship forming… and I definitely wouldn’t repay her as the difference (what she tried to cheat me out of) would be subtracted from the total (that she gave me).