When any special fund of a public utility department of a town has retired all bond and warrant indebtedness and is on a cash basis, if a reserve or depreciation fund has been created in an amount satisfactory to the state auditor and if the fixing of the rates of the utility is governed by contract with the supplier of water, electrical energy, or other commodity sold by the town to its inhabitants, and the rates are at the lowest possible figure, the town council may set aside such portion of the net earnings of the utility as it may deem advisable and transfer it to the town's current expense fund: PROVIDED, That no amount in excess of fifty percent of the net earnings shall be so set aside and transferred except with the unanimous approval of the council and mayor.