Right, it could be an auto entry, but this gets triggered on some event. click on the settled amount and check date and see if any amount received on that date, or if there is any advance deposit or opening balance available in customer account and the invoice net date is reached and got adjusted automatically.
Anyhow, please wait you will get the expert advice in the forum very soon.
Because the Customer had a credit balance, either you had received an advance payment, overpayment or perhaps another Customer’s payment was post to the this Customer.