No order or indebtedness of a previous year may be paid out of the funds of a fiscal year, unless the funds available from taxation and other sources are estimated to be in excess of requirements for the current fiscal year. If the available funds are in excess of the current fiscal year requirements, an appropriation may be made for this purpose to the extent of the excess, or unless, after the close of the fiscal year, it shall be ascertained that the funds appropriated and available for the fiscal year are in excess of the amount required, in which case the surplus may be applied to the former orders of indebtedness.