The bug ID or problem report number is 18409. Title is 'The payment number is not reserved in Receivables Management' message. This applies to all versions of Great Plains eEnterprise including 9.0.
When you create a sales invoice from an order with Credit Card, Check or Cash payment and try to post the document, you'll receive an error in the sales edit list - 'The payment number is not reserved in Receivables Management'.
MBS would send you a fix by deleting & recreating RM00401 but it only worked once in my case. The solution would be to:
1) Remove the sales payment on the invoice then Save
2) Enter the same sales payment on the invoice then Save
3) Delete the General Ledger entries because when the sales payment was entered the first time the entries were also posted. If you post the sales payment distribution entries, this would create a duplicate entry
NOTE: If you're trying to remove a sales payment on a previous year, make sure you open the Financial & Sales module on that month/year.