Differential invoicing: external document number not available

What causes this issue?

You have defined checks for external document numbers in Customizing.
However, there is no external document number in the credit item of the
IDoc or SOA message.

System Response

The document was not posted.

How to fix this error?

Either deactivate the external document number check for your partner in
Customizing for <LS>Materials Management</>, under <LS>Logistics Invoice
Verification</> -> <LS>EDI</> -> <DS:SIMG.OLMROMRY>Enter Program
Parameters</> or ask your supplier to enter the external document number
required in the credit memo item of the IDoc or SOA message.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.