@Tut What @munir.shafi suggesting is more control over purchase orders. They want to be able to match and link purchase orders to supplier invoices to know what has or has not been invoiced as well as to grn which would give information with respect to whether they still owe us goods or if we owe them for goods that they did not invoice.
As per my understanding, Manager currently keeps track of monetary value outstanding with suppliers, what is being asked is to keep track of outstanding quantities as well.