When billing the customer from the Porta Billing engine you need to manage your sub customers balance. This can be done by navigating to the account and selecting the icon next to the Balance

Once you click on the ICON the balance adjustment wizard will open. From here you can select what type of adjustment needs to be made.

Once the correct option has been chosen you can click on the continue button. This will allow you to enter an amount and add a visible comment and an internal one.

You can then review your transaction and click on the finish once ready.

This will then load the payment, charge the customer, etc. depending on which balance adjustment was done.