Instead of cancelling a media campaign or a campaign item within the campaign you also have the option to cancel just one invoice or invoice item:

Invoice

What happens is that the

1. status of the invoice is changed to Cancelled and
2. a new preinvoice is created with the negative amount for you to issue a credit note.

...

This preinvoice has to still go through the regular billing run. 

Invoice Item

Prerequisite

1. Linear Distribution OFF
2. Billing Category equals: Billing Category "CPM", "CPC", "vCPM" or "CPCV

Open the related list of Invoice Items for you Invoice in View All mode:

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Click on the item you like to cancel and then hit the Cancel Item Button. You will receive a success or error message. Then open the media campaign to view the newly created preinvoice for this one item:

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Next continue with the 6.2 Billing Run