Negative balances mean the customer has overpaid compared to what you have billed them.  In in case it would be unusual to have open invoices.  Instead there are likely open/unapplied payments due to the over payment state the customer is in.

If the balance is incorrect, fix it by adding an invoice and then applying the payment to it.

If the balance is correct then there may be nothing to do, or you may want to issue a refund check coded to AR to bring the balance to 0.00.