If the taxpayer makes any clerical error which is apparent on the face of the return in computing the tax accessible against him, the City Treasurer shall correct such error or reassess the proper amount of taxes, and notify the taxpayer of his action by mailing to him promptly a copy of the corrected assessment, and any additional tax for which such taxpayer may be liable shall be paid within fifteen days after the receipt of such statement. If the amount already paid exceeds that which should have been paid on the basis of the tax so recomputed, the excess as paid shall be immediately refunded to the taxpayer upon the requisition of the City Treasurer to the Mayor who shall issue a warrant on the Treasurer, which shall be payable out of any funds available for that purpose. The taxpayer may at his election, apply an overpayment credit to taxes subsequently accruing hereunder.