The excess by which any special assessment proves larger than the actual cost of the improvement and expenses incidental thereto, and principal and interest on bonds or other evidences of obligation issued therefor, may be placed in the general fund of the city if such excess is five per cent or less of the assessment, but should the assessment prove larger than necessary by more than five per cent the entire excess shall be refunded on a pro rata basis to the owners of the property assessed as shown by the current assessment roll of the city (and to the city to the extent it has been assessed for such improvement). Such refund shall be made by credit against future unpaid installments 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.