Sbi Credit Card - SBI charging fine unnecessarily

I had made a transaction of Rs *,*** on my SBI credit card on Sep **'****. I partially cancelled it and got a refund of Rs **** on Sep **th. My bill generation date was Sep **th. I paid the total outstanding amount - Rs ****. In my October month's statement, I have been charged an exorbitant fine of Rs **** for not paying Rs****. When I contacted customer care, they replied that I should have paid the entire outstanding amount and that they would have adjusted the refunded amount in the next bill. I really don't understand what difference it makes between a refunded amount and an amount that I directly pay into my credit card account. The bottom line is that the dues should be paid within the deadline. Why does sbi have to be bothered about who has paid it? I find this highly unacceptable and expecting SBI to address my concern at the earliest.