In the tradeinterop portal it is also possible to create a credit invoice. Proceed as follows:
Click on 'Invoices' in the menu and then on 'Create new'. This will bring you to the screen where you can create a new invoice.
Just like for a normal invoice, you fill in the required data. To create a credit invoice, enter '-1' under 'Quantity'. The total amount becomes negative when you click outside this field.
In the column with data at the top right you can see that a mandatory field has been added, this is 'Reference to debit note'.
When you have filled in this field correctly, you can save the credit invoice and then send it.