Different PO number at header level and item level

Hi Experts,

I have one requirement wherein user asking to have different PO number at header level and item level in VA01. When I enter PO number at header level the same being copied into item level also at order data tab. But user wants not to copy the same PO number .Both fields are populating in VBKD table IN BSTKD field. Could please advise me to how to stop this?