System will always take the actual GI date as default billing date in VF01--in your case it has to be delayed to 2 days---so insert this logic to your Abaper in Exit ""V05I0001"""(exit for updating billing index table).
Also you can check with USEREXIT_FILL_VBRK_VBRP.
Hope this helps.If any thing wrong--pl update...
Phanikumar