Differential invoicing: external document numbers are inconsistent

What causes this issue?

You have defined checks for external document numbers in Customizing.
The external document number &V1& from the previous invoice, which is
referenced in the credit item of the IDoc or SOA message, does not match
the external document number &V2&.

System Response

The invoice cannot be posted.

How to fix this error?

Ensure that the credit item contains the correct external document
number.
Either deactivate the external document number check for your partner in
Customizing for <LS>Materials Management</>, under <LS>Logistics Invoice
Verification</> -> <LS>EDI</> -> <LS>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.