5 USC 5502 - Unauthorized office; prohibition on use of funds

(a) Payment for services may not be made from the Treasury of the United States to an individual acting or assuming to act as an officer in the civil service or uniformed services in an office which is not authorized by existing law, unless the office is later sanctioned by law.
(b) Except as otherwise provided by statute, public money and appropriations may not be used for pay or allowance for an individual employed by an official of the United States retired from active service.