All contracts made by any school district of the first class shall state therein on what item in the annual estimate of school expenditures the same is based. Every contract before becoming valid shall be properly certified by the school controller, who shall, at the time of so doing, charge up the amount of any such contract against the item in the annual estimate on which the same is based. No contract shall be certified by the school controller if the amount to become due thereon shall exceed the item of expenditure on which it is based.