You will have 3 invoices at the end of this procedure. The 1st invoice is the original one which you are unable to access, thus the reason for this procedure. The 2nd invoice will be for a non-taxable MISC item. The cost of the item will equal the subtotal of the original invoice. The 3rd invoice will actually be a Return Authorization or Credit Memo.
To begin:
1) Insert a new order for the stock item MISC, quantity 1 Make the price of the MISC item the same as the subtotal amount from the original invoice. Turn the order into and invoice. Make sure the invoice doesn’t have any tax. Post the invoice.
2) Commands > Credits & Returns, RA’s. Insert a new RA (Return Authorization) or Credit Memo for the MISC item. Quantity -1. Make sure it has tax. Post the RA or Credit Memo.
3) Commands > Apply Credit. Apply the credit you just made in step 2 to the invoice you created in step 1.
The Invoice you created in step 1 now has the tax amount as a credit.