The amount outstanding reduces the balance of the booking because Preno does not manage debtors.
When an invoice payment is added to the booking, the amount outstanding for the booking is reduced by the total of the invoice.
Preno doesn't know when an invoice has been paid once it has been created. Once an invoice is created you will need to manage payments outside of Preno.
This can be done by managing your invoices in Xero or by checking your bank account for the payment.