Reference to this work item "Issue 424063 "

Description below

When invoice credit only RMA. If the invoice directly error message will come out. If you try to add Serial Number when posting an invoice, a warning message will show up and not allow save Serial Number.

Investigation result:

1. The SerialSales controlled item needs to be registered with Serial Number.
2. The return order with Credit only Disposition code will result in "Scrap = Yes" as default value on the sales order. But it could be changed by the customer.
3. If Scrap=Yes, the inventory transaction will not be generated, but current design still need serial number registration when doing Invoice.
4. Although the serial number registration shows Registered=OK, the registered transaction will have empty InventTransOrigin which causes the warning message. 
5. And finally, it will cause invoicing to fail as it's using newly generated InventTransOrigin during the invoice phase to find the registered transaction thus it could not be found.

Workaround: Set Scrap=No on sales line then doing serial number registration, the inventory transaction will be generated and referenced in registered trans. But this means Scrap=Yes derived by Credit only disposition code is not reasonable.

Solution: Take Scrap field on Salesline into account. When Scrap=Yes, do not check item serial number registration as there are no inventory transactions anyway. But the salesparmline also has Scrap field which could be changed by the customer when invoicing, this Scrap field will not be respected as it doesn't impact inventory transaction generation at all.

Not supported into standard Dynamics right now, but in the scenario of our customer the return order of serial number managed items are critical.

Need this fix asap ( next release )