Hi Noor,
Instead of creating manual bill, could you able to post a credit FICA document as the once off credit?
You can have a specific Main/Sub combination for that. And during posting of that credit you can put a clearing restriction 8 on that. That would prevent it getting cleared with BB debit items. You can easily achieve that by enhancing FQEVENT 42.
Once you do that, until invoice is posted, in FPL9 you would see that as open credit item with restriction 8 and won't clear any BB debit documents.
Later you can let it offset during invoicing. For that you have to maintain the Main/Sub in the configuration of "Item selection during invoicing account maintenance" and "Item selection during bill print out". They are the tables TE514 and TE529 respectively.
This way you would be able to display it in the print invoice and allow it to get offset with the invoicing debt (of course as per clearing rule).
Hope it helps. Let us know how you go.
Regards,
Bodhisattwa