Stop Billing creation till PGI is Done

Dear Gurus,

I would request you to please help me in this configuration, we have a process where a STO is created and converted into Delivery (VL10B) and then we do Proforma Billing (VF01 with respect to Delivery) and after that we create Excise Invoice, in the current process we can create the Billing document as soon as we create a Delivery Document through VL10B without doing PGI, now the client wants that the Billing should not be allowed till the time the PGI is completed. please suggest how to configure this Requirement.

I tried making changes in the copy control VTFL of Delivery to Billing where i changed the Copying routine 009 to 003 at header level and 010 to 004 at Item Level but its giving an error that Item is not relevant for Billing while creating Billing Document.

Thanks in Advance

Ameet Singh