301-71.307—How do we collect the amount of a travel advance in excess of the amount of travel expenses substantiated by the employee?
When the outstanding advance exceeds what you owe the employee, then the employee must submit cash or a check for the difference in accordance with your policy. Your failure to collect the amount in excess of substantiated expenses will cause a violation of the accountable plan rules contained in the Internal Revenue Code (title 26 of the United States Code).