Customer material &1: Billed quantity does not correspond to open qty

What causes this issue?

The customer has billed a quantity of customer material &V1& that is
different from the open quantity due for billing.
The billed quantity from the intermediate document (IDoc) is &V2&
The anticipated quantity due for billing is &V3&
No matter what type of IDoc is created for the self-billing document,
the system calculates the quantity due for billing as follows:
<ZH>For invoices</>

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

The quantity to be billed is the quantity of relevant items in the
delivery.
<ZH>For credit memos</> (quantity corrections)
The quantity to be billed is the:
Quantity billed in the self-billing document
<ZK>minus</> the quantity in the related delivery
<ZK>minus</> the quantity in all existing credit memos
<ZH>For credit and debit memos</> (for retro-billing)
The quantity to be billed is the quantity of relevant items in the
invoice.

System Response

The system creates a billing document using billed quantity &V2& from
the IDoc.

How to fix this error?

Inform your customer of the difference so that they can send a
credit memo for quantity corrections.

Error message extract from SAP system. Copyright SAP SE.