Need to be able to sum PR line amounts (for the lines on PO) to compare with PO total

Summary

Need to be able to sum PR line amounts (for the lines on PO) to compare with PO total

Content

Scenario: 

Create a PR with 3 lines:
Line 1: Item A : Total Line Amount = 100 USD
Line 2: Item B : Total Line Amount = 100 USD
Line 3: Item C : Total Line Amount = 100 USD

PR is Approved.

Now in Process Requisition, Buyer will only select Line 1 & Line 2 and create a PO
Once PO is created, buyer would now add another new line as Line 3 which has no reference of PR and the PO line 3 amount is 200. Also the buyer would change the Line 1 total amount to 50 USD which is less than the amount compared to PR line amount.