Query for Block to add Purchase Order as Draft Document

Good Evening  My Dear Friends....

My Scenario :

I set the Approval procedure for Purchase Order Document

I want to Block the PO for going to approval via transaction notification ,if The Purchase order line containing Project and Dist.rule field will be empty.

Note the point:

I can able to Block PO,if it does'nt apply approval,My problem is that,A new window will open for approval applied PO when click on the add button. So,I can't able to manage the Docentry of that PO.

So,I am Expecting helpful query from my dearest friends...