Sales order invoice not showing correct "purchase order price" rate

This is kind of tricky to explain but I will try my best.

We have noticed on multiple occasions that when one of our reps enters a sales order with products at the price level of "purchase order price" upon invoicing the price is incorrect and instead is the part purchase price, not the purchase order price. The purchase order and purchase order invoice are correct but not the sales order invoice which throws off our sales reps margins which effects their commissions. Not sure what can be done about this or if anyone has come across this before. Thanks in advance!