[Resolved]  Idea Cellular — charged against prorated overage

I was a user of idea postpaid service. Due to some reason i decide to quite the postpaid and port to prepaid idea.
Scenario is : my postpaid bill generation date was every month of 4th. It was generated with amount 351.66 rs. I have paid the bill by 352 rs. On the same day i have converted to idea prepaid and also did recharge of 250. I started using idea service immediately on the same day. I have successfully migrated to idea prepaid.
Problem is : idea post paid department send me mail and message regarding the bill generated for the month may is 212.72 rs. As i have not used postpaid service any more from the bill generated and bill paid date which is same day. I have checked the bill invoice and found that charged applied on the prorated overage charged on data. I dont understand if i have not used data after bill paid then how can idea applied such charges.

Idea send me a legal notice from their advocate to pay the amount. If i dont paid then i have to go in front of judge.
Idea acting like poor and chip. If customer want to leave them then they will charged you.

Please help me how should i resolve this thing.