No office, department or agency, during any fiscal year, may expend, or contract to expend, any money, or enter into any contract which by its terms involves the expenditure of money or incur any liability for any purpose, in excess of the amount appropriated for that purpose in accordance with the law and this charter. Any contract, verbal and written, made in violation of this charter shall be null and void. Any officer or employee of the city who violates this section shall be guilty of an offense against this charter and, upon conviction thereof, shall cease to hold his office and shall be terminated. Such officer or employee and his surety, if any, or either alone, shall be liable for any loss the city may sustain as a result of such violation. Nothing contained in this section, 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.