Read more carefully, FB09 is a transaction not a program (update line item, where FB02 is update document), use SHDB to build a small BDC wrapper report (very easy, try it) or use a FM like FI_ITEMS_MASS_CHANGE (which will build a BDC for FB02)
BSID in only a copy (secondary index) of some BSEG items for performance (not cleared, debitor) so update MUST come from BSEG standard updates.
Regards,
Raymond