Not sure if this is D365 product design failure or the consultant's (that we hired) failure. To do a credit memo, we have to put in a vendor invoice with a different invoice number (e.g 123A vs the original invoice 123). The difference between the credit memo and the vendor invoice is the negative quantity vs positive quantity. So when we have to correct invoice 123, we have to create another invoice 123A as a credit memo and then create another invoice 123B to correct invoice 123.