After the budget resolution has been adopted, the council shall have no power to increase the amounts fixed in the budget resolution, by the insertion of new items or otherwise, beyond the estimated revenues, unless the actual receipts exceed the estimates and then not beyond the actual receipts. The council may at any time, by resolution, approved by a majority of its members, reduce the sums appropriated for any purpose by the budget resolution, or by a vote of four members, authorize the transfer of sums from unencumbered balances of appropriations in the budget resolution to other purposes.