41 C.F.R. § 301-51.202

Employees must account for their travel advance after completion of their assignment. The employee must file a travel claim which accounts for the advance in accordance with the agency's policy. If the employee is in continuous travel status or submits periodic reimbursement vouchers on an individual trip authorization, the agency may reimburse the full amount of the employee's travel expenses without any deduction of the advance until such time as the employee files a final voucher.

(a) If the amount advanced is less than the amount of the voucher on which it is deducted, the employee will be reimbursed the net amount.

(b) If the advance exceeds the reimbursable amount, the employee must immediately refund the excess.