Airtel Broadband - Refund of over billing

I have been subscribing to Airtel broadband services since **** and the Plan chosen was for Rs ***/- & has remained at this Plan till date. It is in my wife's name . My billing has been steadily increasing over the years & bill date of **.July.**** is for Rs ***.**. It had come to my notice that there is a "billing error" & hence requested a refund the extra amount at the earliest. I sent them an excel sheet with all Billing details & extra amount charged of Rs ****.**+ extra tax charged on **/**/*****. Complaint reference number was ******** I asked for refund but they said it was not possible & alternatively given a Rs ***/- month reduction in bill for LIFETIME on **/**/**** by email.The terms of my acceptance was for LIFETIME at Rs ***/month deduction. Now since about a week they have started saying that this Rs***/- is a DISCOUNT & same was being withdrawn. I informed them (Appellate) that this was a REFUND in lieu of of the overbilling, but they are saying it is not possible to continue the Rs ***/month refund or reimburse my over billed amount with interest.
What I want: The terms of my acceptance was for LIFETIME at Rs***/month deduction. This offer was made by them & I accepted.