GoAir — amount not refunded

While on board in flight i purchased one veg sandwich and one coffee.in return i gave crew member ₹ 500/- but the total bill amount was ₹ 350/- even she told me that they will pay back my balance amount but didn't. How could i get my money back???. I am in a state of shock how crew members do such types of mistakes. I need my money back asap.
Even on asking them didn't get any response.