Indigo Airlines — Huge deduction on cancellation

Address: 560075

I booked 3 tickets to goa and 3 returning tickets from goa to bangalore with indigo airlines. But i had to cancel all the tickets because the venue of the event in which we were suppose to go, got changed to mumbai. I was suppose to book the tickets to mumbai with the indigo airlines after the cancellation of the tickets but 60% of the money was deducted and only 40% was refunded back to my account. I understand if the cancellation was done few hours prior or few days prior but i cancelled the ticket 3 weeks prior to the date. Isn't 3 weeks an enough time for the cancellation? Is this how they do the business? What services they have provided? There seat will not go empty as there is enough time when people will make the booking.
They charged me rs.10, 666 for the tickets and deducted rs.5878 for the cancellation eventhough the cancelltion was done way prior 3 weeks. These people are simply looting the customers which is unacceptable.

Booking reference number - t5snfl

Numeric booking id[protected]