https://userapps.support.sap.com/sap/support/knowledge/en/2786250
2786250 - QA11/QA12: error M7021 - 'Deficit of PU IR quantity...' | SAP Knowledge Base Article
It is not possible to post stock to 'Return Delivery' / 'Return Posting' in transaction QA11 or QA12, as error M7021 is raised, saying: Incorrect posting back...