When reviewing a sales order, you notice that there is a "negative" balance on the order.
For example, you completed an order in Sales > Daily Sales. The order total was $5.00, and the customer gave you $10 in cash. You completed the order and gave the customer $5 of change. On the order record under Sales > Order search, the balance of the order now shows a value of ($5.00).
This is normal. Using the example above, the ($5.00) balance does not indicate that the order still has an outstanding amount due. It indicates that the customer paid $10 on a $5 order and therefore had change due of $5.00.