A negative balance on the Point of Sale usually indicates that an item was voided, but never refunded the amount. To close the order with the negative balance, you can
Add an Arbitrary Extra Item
1. Tap Open, on the order screen.
2. To add an extra item to an order, tap Add Extra Item on the bottom left-hand side of the order screen. Enter any name, the price (the negative amount left outstanding), and set Tax to 0. This will allow you to keep payment on the order. However, if the customer was supposed to receive their money back, you must use the Refund Payment option.
To refund a payment, bring up the order in question and tap the Pay button. On the payment screen, tap on the Refund Payment found on the right-hand side. Follow the prompts to complete the Refund, and then hit Done.