When you send products, sometimes they are returned. The returned products can then be credited directly to the invoice. In order for this to happen, the invoice must have the status "Pending".
Open the desired invoice, click on the tab "Incoming payments & credits" and click on "Create credit".
Select the option "Select positions" and, below this, the returned product position via the check box on the left-hand side. Specify the number of products that were returned and click on "Save". The credit is then created automatically and posted directly with the invoice.
Please note that if credits and invoices have been settled with one another, the product positions of these two documents can no longer be edited.
The credit that has been created is then visible in the invoice under the tab "Document flow".