An officer or agent of a county or municipality who is charged with the construction, improvement, or maintenance of any building or work, or the management of a public institution, may not enter into any contract that binds or purports to bind the county or municipality to pay money not previously appropriated and remaining unexpended for the purpose of the contract. If an officer or agent of a county or municipality willfully or knowingly enters into or participates in entering into a contract prohibited under subsection (a) of this section: the officer or agent is personally liable for the contract; and the county or municipality is not liable for the contract.