Dear VAL,
As per your requirement you can change the price in the in the billing level .
there are some pre customization you have to follow for this option .
1. as per the below screen shot you have to check the posting for billing block .so that you have to release the account posting manually .
Note - once you released to the accounting everything will be greyed out .
2. as per your requirement you can make your base price condition manual or through the accesses sequence .here you can change the billing date also , it is also editable mode only . you can enter manually or you can update price through update tab.
Regards,
C.B Reddy