117. From the fiscal year beginning in 1972, a municipal corporation or school board bound to adopt a balanced budget shall be exempt from such obligation to the extent that the application of sections 12 to 14, 16, 18, 19, 21, 22, 97 and 104 does not permit the exact determination of the amount of revenue involved when the budget is adopted. Such exemption shall cease from the municipal or school fiscal year beginning in 1974 when the amount of such revenue shall not be entered in the budget at an amount greater than ten per cent of that in the financial statements for the next to last fiscal year preceding that for which the budget is prepared.