Direct invoicing

Hi

There is a requirement for direct postings through FI for invoices to vendors. In FB60 we can not book down payments for stock or assets to vendor with special GL indicator. Users use F-47 & F-48 for making down payments to vendors but there is one issue we are facing . Its that we are unable to book downpayments with reference to PO. There is a field in line items and we want it to be mandatory for all downpayments made to vendor accounts.What should we do meet this need.