Airtel — amount deducted from main balance instead of 2g data pack

i hv data left in my 2g pack and the company deducted 90 rs from my main balance. The question is when the data is not 100 percent utilised then how can they deduct the amount from main balance. Who has given them such an authority? this is clearly fooling around people. the answer given by customer care support was of third grade. the customer care person gave me a logic that when the downloading is in process and i[censored] r running low on internet balance then automatically amount gets deducted from main balance so as to avail uninterrupted service. this is how telecom companies are fooling around and minting money. when i asked him to email his source of information then he diverted my call to someone else who gave me another number to call and enquire. whosoever is reading this complaint should make it a point that people on the other side are not fools. customer care is not for doing namaste n aapka din shubh rahey...it is for providing better services which have failed to provide.
my contact no is [protected]
date 13th may 2014