MakeMyTrip — asking for more money after confirmation from makemytrip

i booked my flight from makemytrip and it was a round trip. It was showing abount of Rs 6855.00/- for the round trip. As it was a lowest fare i was getting so i booked that. I got confirmation from both indigo and makemytrip. But now they are asking for more Rs 6700.00 by telling it a system error. I cant more amount and they are saying taht if i dont give them full payment then my booking will be cancelled. now what to do and please reply for this as early as possible.