If the charge has not been paid against, you can revert the charge and raise a new charge of correct value.
If the charge has been paid against or you do not want to revert the charge, you can issue a credit note to the member for the amount which has been charged extra. If you raise the credit note before raising the invoice, the invoice will reflect an advance payment of this amount (assuming there is no other pending charge / advance payment) which will reduce the total amount payable by the member.