After the budget has been adopted no money shall be drawn from the treasury of the city, nor shall any obligations for the expenditure of money be incurred[,] except pursuant to the budget appropriations. The adoption of a budget shall not be construed as authority to any officer of the city to expend the amount set forth in such budget unless such expenditure has been authorized by proper warrant. The council may transfer any unencumbered appropriation balance or any portion thereof from one department, fund, or agency to another. The balance of any appropriation which has not been encumbered at the end of the fiscal year shall revert to the general fund and may be reappropriated during the next fiscal year.