Credit card_gst charges

HDFC Credit card company says that GST once charged cannot be reversed. So  they have wrongly charged a bank charge and they are ready to reverse it but the GST portion has to be paid by the customer they say. They are quoting that GST component once charged will not be liable for any reversal. I think it sounds illogical. If sale is reversible then the tax on the sale should also be reversible.  Please comment on the accuracy of their claim.