Credit memo qty smaller than PO qty (no GR)

What causes this issue?

The quantity for which you are entering a credit memo is smaller than
the quantity credited to date minus the quantity ordered.
INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX

System Response

The system has found that the <ZG>(quantity to be credited - credit
memo quantity) x order price</> is greater than the tolerance for
quantity variance set in Customizing.

How to fix this error?

Check your entries are correct and make any necessary changes.
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Error message extract from SAP system. Copyright SAP SE.