5 CFR § 550.203 - Advances in pay.
(a) The head of an agency may provide for the advance payment of basic pay, in one or more installments covering not more than 2 pay periods, to an employee who is newly appointed to a position in the agency.
(b) The maximum amount of pay that may be advanced to an employee shall be based on the rate of basic pay to which the employee is entitled on the date of his or her new appointment with the agency, reduced by the amount of any allotments or deductions that would normally be deducted from the employee's first regular paycheck.
(d) An advance in pay under this subpart may not be made to any employee when an agency expects to make an advance in pay to the same employee under 5 U.S.C. 5927 within 2 pay periods after the employee's appointment.