No officer, department or agency shall, during any fiscal year, expend or contract to expend any money or incur any liability, or enter into any contract which by its terms involves the expenditures of money, for any purpose in excess of the amounts appropriated for that general classification of expenditure pursuant to this Charter, and without prior certification of availability of sufficient unencumbered funds by the Director of Finance. Any contract, verbal or written, made in violation of this Charter shall be null and void. 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 or leases for a period exceeding the fiscal year when such contracts or leases are made, when such contracts or leases are permitted by law.