How to get out of paying my mom back?

Okay so i did some damage to someone else's car (scratched bumper) and she paid for it (like 400 dollars) and now that i have a summer job she wants me to pay her back but i need to save the money for a trip i already bought the plane ticket for. How do i get out of paying her back? I already tried to tell her i'd watch my brother over the summer when she worked and that instead of giving me money to pay for a car of my own just not have me pay her back and she's not going for it. Any other suggestions?