Hi InnoV,
I would personally use the discount feature, in this case, to make things easier. Simply create a discount item, and add it as another line on the invoice with a negative amount to zero out the invoice. I'll show you a sample.
Create a discount item:
- Click the ITEM CODE field on the Invoice page and choose <Add New>.
- Select Discount for the Type.
- Enter the details and click OK.
Add the discount item to the invoice like in the screenshot.

I'm including this article for your additional reference: Use discount and subtotal items on an Invoice. If you want to use a credit memo, you can refer to this article: Give your customer a credit or refund in QuickBooks Desktop for Windows.
I'm just a comment away if you need further assistance. Take care!