On or before the fifteenth day following the close of the authorized travel period for which expenses have been advanced to any officer or employee, he or she shall submit to the appropriate official a fully itemized travel expense voucher, for all reimbursable items legally expended, accompanied by the unexpended portion of such advance, if any.

Any advance made for this purpose, or any portion thereof, not repaid or accounted for in the time and manner specified herein, shall bear interest at the rate of ten percent per annum from the date of default until paid.