To delete line item in sales order

Hi Gurus,

I had created sales order and proforma invoice (proforma wrt to the sales order ),my requirement is that i have to delete one line in sales order when I am trying to delete line item in sales order system showing the error msg that item cannot be deleted because of subsequent document(proforma invoice).Can anybody throw some light how to delete the line item in the sales order.

In this procedure I had cancelled the proforma invoice( in VF02 menu bar sleceted completed).

Regards,

kishore.