5 U.S. Code § 5503. Recess appointments
(a) Payment for services may not be made from the Treasury of the United States to an individual appointed during a recess of the Senate to fill a vacancy in an existing office, if the vacancy existed while the Senate was in session and was by law required to be filled by and with the advice and consent of the Senate, until the appointee has been confirmed by the Senate. This subsection does not apply—
if, at the end of the session, a nomination for the office, other than the nomination of an individual appointed during the preceding recess of the Senate, was pending before the Senate for its advice and consent; or
A nomination to fill a vacancy referred to by paragraph (1), (2), or (3) of subsection (a) of this section shall be submitted to the Senate not later than 40 days after the beginning of the next session of the Senate.