At the beginning of each quarterly period during the fiscal year, and more often if required by the Council, the City Manager shall submit to the Council data showing the relation between the estimated and actual revenue and expenditures to date, and if it shall appear that the revenue is less than anticipated, the Council may reduce appropriations for any item or items, except amounts required for debt and interest charges, to such a degree as may be necessary to keep expenditures within the actual revenue. If the revenue exceeds the amounts estimated in the budget, the Council may make supplemental appropriations. The City Manager may provide for monthly or quarterly allotments of appropriations to departments, funds or agencies under such rules as may be prescribed.