extra bill amount

Sir,
II have idea postpaid connection of PMB Grandeur (1.3)1097. As per company I am getting 1 GB free 3G data. I also received regular massages from company that how many data I have used and how many data is available. So I used data accordingly. but in the month of June I received massage on 24th June 2014 that till 23.06.2014 23.59 hrs usages is 969.09 MB and 54.91 MB data I can use. and even on 24th june 2014 I also not use the 54 MB data. my bill end date is 24 th. But in my bill of June 2014 the charge the data from 21st June 2014 on wards.
When I called to customer care they first told me that the excess amount will be revers in your account. but the next day they called me and told that the charges are increased and the excess data amount will not be reversed. I also went four times for conversion of postpaid to prepaid connection but every time they refuse it and told come for net time. pls help me.