Airtel — airtel money - err to make payment. not resolved from their customer care

Address:	Mumbai City, Maharashtra, 400064
Website:	www.airtel.in

Your offer is misleading. It clearly says Rs 100 cashback.
I haven't done any transaction using airtel money till now. I need to pay bill of 421 for a postpaid connection. So I'm eligible for the offer Flat Rs 100 cashback offer if I pay via Airtel money. Sadly, when I try pay via airtel money, it is showing me another offer which has only Rs 50 cashback.

I didn't have so much of an issue with the cashback offer. I tried to pay my bill with airtel money which I added like 5 days ago before making the payment, it says "Debit not allowed". Why? Why can't I use the money which I just added to my account!