How can I waive an outstanding amount on Invoices?

You have the option to mark an invoice as paid also if there is an outstanding amount left because the customer maybe paid not all of it, or he did not pay the dunning fee. If you did not set an corresponding indifference amount on the connector, the dunning will continue if there is an open amount left. This can only be stopped if you mark the transaction as paid. 

To do this, open the invoice and then click mark as paid.

Always waive dunning fees

You also have the option to enforce the dunning for or waive them automatically. In this case, the invoice will be marked as paid even though there is an outstanding amount. This setting can be set on the ISR connector.