§ 307. System revenues; adjustment of surcharge. All surcharge monies remitted to the municipality by a service supplier and all other monies dedicated to the payment of system costs from whatever source derived or received by the municipality shall be expended only upon authorization of the board and only for payment of system costs as permitted by this article. The municipality shall separately account for and keep adequate books and records of the amount and source of all such revenues and of the amount and object or purpose of all expenditures thereof. If at the end of any fiscal year the total amount of all such revenues exceeds the amount necessary for payment of system costs in such fiscal year, such excess shall be reserved and carried over for the payment of system costs in the following fiscal year. However, if at the end of any fiscal year such E911 reserved fund balance exceeds an amount equal to five percent of that necessary for the payment of system costs in such fiscal year, the board shall by local law reduce the surcharge for the following fiscal year to a level which more adequately reflects the system cost requirements of its E911 system. The board may also by local law reestablish or increase such surcharge, subject to the provisions of section three hundred three of this article, if the revenues generated by such surcharge and by any other source are not adequate to pay for system costs.