Expense reports with a credit balance

We have occasions where employees will submit and expense report which has a credit balance (they owe money back to the employer, usually for an advance which was too much). The balancing line added automatically to the report is costing the credit back to a certain budget string which differs from the usual balancing budget string. Can anyone point me to where the setup for this is? We need to update one of the segments in the string. Thank you!