HDFC Bank — unjustified finance charge and false assurance of reversal

I am extremely upset with the way HDFC treats a customer who has been with them for years, the way they handle a complaint and reject a justified request. In the month of August, the due on my credit card was approximately 129000. Accordingly, I paid 1lac within the time given, whereas 29 thousand reached them three days after the due date due to some net banking error. Surprisingly, when I received the September bill, I saw a finance charge of more than 3000 for a 3 days’ delay!! I immediately called the customer care, I was told it would be reversed since the delay was only 3 days, it did not. I called again on the 18th of September, in which I was assured by a representative that it would be reversed since she had spoken to the supervisor and that in 2 days I would get a confirmation message. I waited, the message never came. I called back on the 23rd as my payment was due for the next month. I again got the assurance that it would happen. I even managed to record a little of the conversation. Based on that assurance, I made a payment of INR 21138 on the 25th of September. Today, I received a message that my request for reversal could not be processed. After all the guarantee, the assurance from their end, they fooled me into making this month's payment and after the payment for this month is received, I immediately received a message that the reversal could not be processed!! I can’t believe that they could cheat a customer to this level. I am all set to raise this issue through social media if required, but I will not tolerate such false commitments, that too on a complain /request which is justified.
Request you to help me in this.