Immediately following the close of each month the council shall requisition the county commission for the estimated amount of the month’s expenditures and within ten days after filing the requisition, shall submit to the county commission a certified, itemized statement of all expenditures covered by the requisition. The requisition shall constitute the basis for immediate issuance by the county commission and it shall, if there be funds available therefor, promptly issue a warrant covering the requisition in full and drawn in favor of the treasurer of the council. For the purpose of this section the chairman and secretary of the council shall be regarded as certifying officers. The requisition for any given month shall not exceed one-twelfth of the total amount appropriated for the year unless a reserve shall have accumulated as a result of expending less than the aforementioned twelfth portion during one or more preceding months, in which case the reserve shall be constantly available for current expenditures. If the amount of the certified itemized statement is less than the amount requisitioned the difference shall be credited against the next requisition. Any unused funds remaining in the appropriation on December thirty-first shall revert to the county treasury.