If you are importing supplier invoices there will come a time when a product is on that invoice that is not yet in your CR system OR the data in your system doesn't match with the invoice.
There is a "Supplier Invoice Import Report" that you can run that will show you all mismatches. You can then correct these mismatches by either:
a) Creating the product or
b) Updating the supplier reorder number (this is how CR matches data to the supplier's invoice)
Once you do so you can drill down into the PO from the report and click the "Reprocess" button shown below, and the system will reprocess the EDI file and update the broken links. Even if some time has passed in between when the product was received and today, past sales transactions will be automatically updated with the appropriate cost going backwards based on the effective date/time of the PO.