Your receiving staff has a file of open purchase orders on hand, and matches the purchase order number against the number on each incoming delivery. This step ensures that some level of authorization was granted for each delivery. But does the receiving staff verify that all items were received, or do they just accept the quantities already printed on the purchase order? If the later is the case, then the company may be paying for more items than it actually receives.
One way to ensure that the receiving staff actually conducts a receiving inspection is to only issue them a purchase order with all quantities blanked out, and force them to fill in the actual quantities received. Since this may unduly increase their work load, at least consider blanking out the quantities ordered for more expensive items. This approach also works if the receiving staff accesses purchase order information on a computer, simply by altering their viewing screen to not show quantities ordered.