The corporate authorities may delegate authority to heads of municipal departments, boards, or commissions to delete, add to, change or create sub-classes within object classes budgeted previously to the department, board, or commission, subject to such limitation or requirement for prior approval by the budget officer or executive officer of the municipality as the council, upon a two-thirds vote of the corporate authorities then holding office, may establish. By a vote of two-thirds of the members of the corporate authorities then holding office, the annual budget for the municipality may be revised by deleting, adding to, changing or creating sub-classes within object classes and object classes themselves. No revision of the budget shall be made increasing the budget in the event funds are not available to effectuate the purpose of the revision.