At the beginning of each quarterly period during the fiscal year, and more often if required by the Council, the Mayor shall submit to the Council data showing the relation between the estimated income and expenses and actual income and expenses to date; and if it shall appear that the income is less than anticipated, the Council may reduce appropriation, except amounts required to meet contractual obligations and for debt, interest, and other fixed charges, to such a degree as may be necessary to keep expenditures within the cash income.