I had faced this same issue with SBI Cashback card couple of months back. Bill amount was 53k, and there was a refund of 10k after bill date but well before due date. I paid the balance 43k plus some 1k extra before the due date, but they still charged me fin charges. I raised a complaint quoting the RBI regulation, but they refused to reverse the charges. Escalated to NO, PNO also threatening to complaint to RBI, still same stupid copy paste answer. After that I was about to complain with the RBI Ombudsman but saw that there's another CS Head for escalating the query, so thought why not check if atleast these guys have any idea about RBI regulations before going to the Ombudsman.
Unsurprisingly, nothing was different. One person called and explained that as per internal policy refund is not considered as payment and that you have to make the payment of total amount due. I told that your internal policy is fine, but what about RBI regulations, is it not applicable to you? She was again repeating same dumb thing and responding as if she didn't even hear the word RBI. Whatever I ask, she says the same thing again. I told, understood and cut the call.
Raised a complaint with RBI after this, in one week time entire finance charges plus GST was reversed. No questions asked.
So, please don't listen to these stupid SBI customer care people, PNO, CS Head and what not, just looting people's hard earned money. Raise a complaint with the Ombudsman.