No officer, department, or agency shall during any budget year, expend or contract to expend any money or incur any liability, or enter into any contract which by its terms involves the expenditure of money, for any purpose, in excess of the amounts appropriated for that general classification of expenditure pursuant to this Charter. Any contract, verbal or written, made in violation of this Charter shall be null and void. Any officer or employee of the City who shall violate this section shall be guilty of a misdemeanor and, upon conviction thereof, shall cease to hold his office or employment. Nothing in this section contained, however, shall prevent the making of contracts or the spending of money for capital improvements to be financed in whole or in part by the issuance of bonds, nor the making of contracts of lease or for the services for a period exceeding the budget year in which each contract is made, when such contract is permitted by law.