A time limit for payment shall in principle be deemed to have been observed only if the full amount of the fee has been paid in due time. If the fee is not paid in full, the amount which has been paid shall be refunded after the period for payment has expired. The Office may, however, in so far as this is possible within the time remaining before the end of the period, give the person making the payment the opportunity to pay the amount lacking. It may also, where this is considered justified, overlook any small amounts lacking without prejudice to the rights of the person making the payment.