Hi,
Normally during the process of Services , when you post the SES and Accept it system will post the GR quantities and you will perform a Invoicing at MIRO for the SES and in your case it is ERS . But when ever there is a credit memo received , for these POs credit memo will be posted and system will be able to revoke the SES entered for change .
Here your issue is ERS . In the normal PO , when you post the 122 for the GR system feels a credit memo neccessary and when you run ERS it posts a credit memo . But the in the services case , you are posting a credit memo first ( which has to be auto ) and after that you are changing the SES .
Please refer to the below links .they are useful .
Service entry sheet and Credit memo
Modify/revoke Service Entry Sheet when LIV is posted and quantity blocked
Regards