Purchase Order vs. Bill Amount Report and Deviation

Hello, 

Is there a way to compare purchase order amounts vs. actual bills. 

Here is the workflow I am looking to run. 

Project Manager enters a purchase order (to a vendor) (with the customer specified) 

Vendor sends back an invoice 

Project Manager approves the invoice 

Bookkeeper converts the purchase order to a bill (that was created by the Project Manager) 

Bookkeeper makes any adjustments to amounts (if there is a deviation from the original purchase order) 

(((That deviation is what I am trying to track on a customer basis and on an aggregate basis)))

FROM THAT POINT:

Is there a way to compare the deviation between the converted bill (that started as a purchase order) and the actual bill itself?

So we could track the variation for project? 

Also, is there a way to see this on a bill/purchase order invoice basis, rather than looking line item by line item? 

Thank you in advance for the help!

John