Sales order with billing plan - Billing plan to be included in incompletion

Dear Friends,

I have sales order with item category configured for periodic billing plan, and invoice is order related. User can give the price in the SO with a condition type, but may not enter the billing plan. As a result this orders will not appear in the billing due list and billing is missing in these cases. I want user to enter billing plan and without it order should not get saved.

SAP standard does not give billing plan in incompletion for sales order item.

Your suggestions pls

Regards,

Aju