Hi Vanessa,
I know this won't be a complete help but did you check the 'EXTENSION1' parameter in the BAPI? Following has been mentioned in the documentation of the BAPI.
Container for 'Customer Exit' Parameter
Description
This exit can be used to edit the accounting document before it is transferred to the accounting components for the update.
For this, BTE technology ( Business Transaction Events) is used.
Regards,
Aniket