This has been an issue for at least 3 years.  Submitting suggestions to developers clearly doesn't work or this would have been fixed.  Having credit memos and applying them to new invoices is such a fundamental part of invoicing.  It appears that the development team does not understand, or just does not care to address, obvious flaws and important upgrades that would actually improve the fundamental operation and usability of your product. It shows a total lack of understanding of the way your product is used "in the real world."