Subcontracting Purchase Order


I have done customization with purchase order and trade aggrement that in po lines items drop down will display item list from Trade aggrement only. That is item which is present under that respective vendor’s trade aggrement that only will get fetch in po lines item drop down list. I am using subcontracting also in AX 2009. I am adding Production order of vendor type then on estimation it generates PO automatically. This time my item is not present in trade aggrement then it is directly taking rate in po from item master. I want to avoid it. How to do it?

Please help!