Differential invoicing: Item &1 is inconsistent

What causes this issue?

If a preceding document has been posted, a differential invoice must
contain exactly one credit memo item corresponding to and assigned to
each invoice item.
A credit memo item must contain a valid reference item number that
refers to the assigned invoice item number. There must not be more than
one credit memo item referring to the same invoice item, and there must
not be more than one invoice item used for the same goods receipt.
If no preceding document has been posted, there must not be any credit
memo items. In that case the invoice item must be a positive amount.

System Response

The invoice cannot be posted.

How to fix this error?

When performing differential invoicing, ensure that the IDoc or SOA
message includes the correct invoice items and credit memo items.

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.