SAP note: 1747358-ML81N:Service Entry Sheet(SES) acceptance can be revoked,despite invoice already posted.
Symptom
SES(ESSR-KZABN)can be revoked,even though an invoice has been already posted against it. When try to revoke the SES, awarning message1388 An invoice already exists for this entry sheet' is issued, if you ignore this warning, SES will be then revoked.
You want to change this message to an error message to prevent the action.
Resolution
To prevent SES being revoked in case it has follow on invoices,please go to transaction OMBZ,and remove the flag Reversal of GR allowed for GR-based IV despite invoice' for movement type102. In case the flag is not set for movement type102,
when you revoke the SES, even you ignore the warning message11388,error M7021will be issued later so that the SES cannot be revoked.