The only way to remove the invoice would be to either delete it or raise a credit note against it. You could in turn put a note on the credit note explaining the reasoning.
What you may also want to do is move the balance of the stock (the dog food) to another nominal such as 6202 Gifts and Samples, similar to this:
It’s worth noting that without them giving you an invoice it will show as an expense on your accounts.
However, I’m not an accountant, so I recommend double-checking with one to be sure.
Hope this helps.