No person designated by the Secretary, or designated pursuant to an agreement under this subpart, as a certifying officer, shall, in the absence of gross negligence or intent to defraud the United States, be liable with respect to any payment certified by him under this part.

No disbursing officer shall, in the absence of gross negligence or intent to defraud the United States, be liable with respect to any payment by him under this part if it was based upon a voucher signed by a certifying officer designated as provided in subsection (a).