If the finance director, upon investigation or upon checking returns, finds that the fee or tax paid on any of them is more than the amount required of the taxpayer, he shall refund the amount overpaid by a warrant upon the general fund. If the finance director finds that the fee or tax paid is less than required, he shall send a statement to the taxpayer showing the balance due, who shall within three days pay the amount shown thereon. [Code 1980 at § 15.04.130].