In the event that any special assessment proves larger than theactual cost of the improvement and expenses incidental thereto, such excess 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 then owners of the property assessed. Such refund may 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.