Ability to edit quantities on POs to levels below that in an order that originated from a web users Purchase Requisition Merged

We have found that it isnt possible to edit the quantities on a PO below that which was entered on the original requisition when raised by a web user. Therefore if eventual accepted final quantity is less than that originally entered, we either have to amend the amount per item to make the total the same, or we have to re-enter the line. This commonly occurs on orders relating to catering where we might be quoted a per head charge at a certain quantity but only pay in the end for the final number who were catered for. Please can this be addressed so that desktop users can reduce the quantities below that on the original purchase requisition.