ML81N - Exit before saving data in ESSR table

Dear All,

I have a requirement for ML81N to validate entry sheet external number(ESSR-LBLNE) while saving the transaction.

I have tried with include LMLSRF0F but it is working after saving the data in ESSR table .

Also I tried user exits-

SRVESLL - It contains data of ESSL and not of ESSR.

SRVEDIT- Screen elements are not working proper, 'service sel' button is not appearing on screen.

SRVESSR- It is working on create event before entering data on screen, not on save.

Please suggest any exit which will work at the time of saving so that user should not be able to save the transaction with error message.

Any other alternate options are also appreciated.

Thank you.