Excellent.
So I would assume you run FKKINV_MA for invoicing.
Here are the table names:
Invoicing header: DFKKINVDOC_H
Invicing item: DFKKINVDOC_I
FICA documen number (as OPBEL) referring invoicing document number(as XBLNR):
CA AR/AP table: DFKKOP
Header table: DFKKKO
Also, for your reference below are table data for billing document (which is the source document for invoicing):
Header: DFKKINVBILL_H
Item: DFKKINVBILL_I
Hope it helps.
Please let me know if you want more information.
Thanks,
Bodhisattwa