Hi Ptonjalken
Mostly, this has to do with your invoicing process and something called the matching principle. Even if you send invoices ahead of time, that isn't counted as revenue until the work or related expenses occur. Is there a reason to send out invoices so early? Do you require deposits up front on work? (there is another, better way to handle this).
Now that you find your self in this situation, you'll need to void the invoice and related taxes. As you've indicated, a credit memo is the best way to do this. From the menu, click the +New button, and under the Customers column, select Credit Memo. Enter the value of the original invoice, marking it appropriately for tax - inclusive or exclusive, and selecting the correct tax code.
Once you've saved your credit memo, you should see the taxes show up as a credit on your tax report. Don't forget to mark the invoice 'paid' or settled against the credit memo.
From the menu pick Invoicing and Invoices find the invoice, and select Receive Payment and you should be able to allocate the invoice and credit memo against one another.
Hope this helps!
