Please check the below option.
Maintain an access sequence such that the billing type field comes above the current access that will be used for sales order level pricing.
meaning. lets assume that at sales order level, the condition type will be fetched through Access sequence say - Sales org/ Sales docucment type.
lets create an additional access for this condition type as say - sales org/ Billing document type(with exclusive indicator checked)
we will maintain the actual value on Sales org/ Sales document type & maintain zero against Sales org/ Billing doc type.
if we have price re-determination ( B/C) that can happen through Copy control at the actual customer invoice level, the price re-determination will then read zero against this condition type for the customer invoice.
& for copy control between order to Proforma we will indicate price redetermination as say unchanged (D/G). this way price at proforma will match the Sales order.
the above suggestion is purely on the standard parameters. if this is not ok we can go with Routine at the pricing procedure level against this condition type.
Regards
Ilango