Tata Docomo — Billing Charges for bill aug 2013

I am using Tata docomo CDMA Card for last 4 yrs. Recently i have purchase new HTC phone after that i try to start internet on my mobile but it's not working. I reach to TATA Docomo Outlet for help they suggested me that you have to change your sim because you are using old sim, however to use internet on your phone u need to change with OHM card. I do that, i ask executive if any internet plan available he told me to activate with 43 internet plan you will get 2 GB Internet usage free. i do that on 26-Jun-2013. After that on 19-Jul-2013 i received bill of as per my billing cycle in that i didn't found any problem.Bill amt 747 which includes also charged 30 charges for pack on prorate basis. After that in next month(August), i received bill on 19-aug-2013 amt 2735 which includes Internet data usage charges (1735.00) + normal call charges + 43 data pack charges. I have register complain regarding bill, now they are saying 43 plan was not applicable to you because you have used High speed internet.i told him if that is not applicable than why plan was activated on my mobile. Now they are saying that when you activated plan that time our system don't now which mobile you are using. i ask when you generate bill at that time you came to know that still why this plan activated on my mobile than they are saying you need to deactivated. It means if my mobile is not eligible for that plan than also i have to bear with all charges.i ask them because of your system mi stack, i have been charged why should i pay for this but they are not ready to except their mistake. please provide me solution for this.