At or before the first meeting of the year in which said budget is to become effective, the Council shall adopt an appropriation ordinance in which appropriations shall not exceed the estimated revenues of the municipality. Such ordinance may provide for an interim appropriation, but in such event an annual appropriation ordinance shall be adopted not later than the time provided by General Law. Such appropriation may be amended as necessary from time to time by ordinance, but at no time shall the total budget as certified be exceeded.