Sales Order Creation and Buying Agreements

I am not sure if anyone has had this issue.  When setting up a buying agreement, there is a space to enter a PO Number.  This from what I can see is good if you have a blanket PO to cover the period of the buying agreement, which in our case is a calendar year.  We do not have blanket Purchase Orders set up on buying agreements. 

So when entering a Sales Order a PO number is entered at the header level.  The problem is that when the order is processed, the system checks for a PO number on the buying agreement.  If it is blank it overrides the PO number entered at the line level for all line items for those that exist on the buying agreement.  Items not on the buying agreement inherit the PO number from the header level.