State Bank of India - SBI cards charges unacceptable finance charges

I had made a transaction of Rs **** on my SBI credit card on dt. **/**/****. I partially cancelled it and got a refund of Rs **** on dt. **/**/****. Also I had made a transaction to amzon of Rs ***** on dt. **/**/****. I cancelled it and got a refund of total amount Rs ***** on dt. **/**/****. My bill generation date was May **th. I had paid the total outstanding amount - Rs ***** excluding refunded amount before due date **/**/****, I have been charged finance charges of Rs. ****.** and GST ***.** for not paying total amount due.
When I contacted customer care, i dropped mail to customercare@sbicard.com *-* times how they charged but they are just giving definition of finance charges rather than telling why i am charged and even after escalating to nodal officer there is no resolution.
After that same happen to nexe billing statement Dt. **/**/****. I had paid the total outstanding amount - Rs ***** before due date **/**/****, I have been charged finance charges of Rs. ***.** and GST **.** for not paying total amount due.
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. SBI has looting custmor. expecting SBI to address my concern at the earliest related both statement.