The excess by which any special assessment proves larger than the actual cost of the improvement and expenses incidental thereto may be placed in the General Fund of the City if such excess is five percent or less of the assessment, but should the assessment prove larger than necessary by more than five percent ,the entire excess shall be refunded on a pro-rata basis to the owners of the property assessed. Such refund shall be made by credit against future unpaid installments in the inverse order in which they are payable to the extent such installments then exist, and the balance of such refund shall be in cash. No refunds may be made which contravene the provisions of any outstanding evidence of indebtedness secured in whole or part by such special assessment.
(1981 Code § 1.172)