Middleware mapping of custom field in Customer

Middleware mapping of custom field in Customer

If field names are same in KNA1, BSS_CENTI (in CRM and R3), BSS_CENTIX (in CRM and R3), then standard code automatically sends data into Bdocs.

Try checking CRMSUBTAB table in CRM and R3 if there are any modules setup.

Check exits - TBE34 table - See below links
https://wiki.scn.sap.com/wiki/display/ABAP/BTE+-+Business+Transaction+Event
https://scn.sap.com/thread/524466