Hi Das,
You shall use this event DE_BALE as this is called before the batch processing. You have to develop a Z FM based on the sample interface SAMPLE_INTERFACE_DE_BALE. For testing the user exit, you have to create a project in TBE24 and register the same in TBE34.
Please find the guide for BTEs. The author has illustrated it with Process Modules. In your case, it can be done in P/S Modules.
BTEs - Business Transaction Events
Regards,
Karthi M R.