airtel - Refund of balance money after Porting Out

I ported out of Airtel Post Paid subscription.
I paid complete monthly bill amount in advance to avoid rejection though I applied for porting in the first ** days of billing cycle.
Porting was completed and there is a balance amount as porting was completed in the middle of billing cycle.
I received invoice from Airtel with following details:
Relationship Number: **********
Bill statement: **.**.**** - **.**.****
Due on: **.**.****
Due Amount: ₹ -***.**
I replied to airtel in the same email for refund. They asked for bank account details and I provided same. I received reply saying ticket was created and amount would be credited to my account in * / * weeks.
After * / * weeks, I checked my account as there is no credit, I emailed airtel to know the status. I received reply to check account after a week as payment is initiated.

After a week, nothing was credited and I emailed airtel again. They asked for bank account details again. I emailed the same but now the previous email history with ticket number got deleted.

Then I wrote to Tamil Nadu Apellate. I received reply saying airtel is working on this issue on priority. I asked for deadline. No deadline was provided.

I started copying another email id as I am afraid airtel may delete the email history.

Then I wrote to Tamil Nadu Apellate and Tamil Nadu Nodal officer, with copy to Airtel CEO and Airtel Compliance Office.
Now also I received same reply saying airtel is working on this issue on priority. No deadline was provided.

Please do the needful to get refund from Airtel