Purchase Order

We have a purchasing agent that was trying to update the quantity on the purchase order to match the invoice.  The invoice was already attached when she tried to make the quantity change to the purchase order.  She tried to make the change and approve it.  The error message that appeared said and could not because the shipment quantity and the purchase order quantity was different.  She went to update the shipment quantity and the field could not be edited. Meanwhile this purchase order status says requires approve, but she can't approve it because the shipment quantity.  She has tried