Encumbrance / Reserve At Completion Flag / PR and PO

We are going to use encumbrance, our concultant did the setup on test system.

While we tested it, it's working on PR corectly. (one person is for entry and one personis for approval) entry person create PR and send approval, if there is no encumbrance the PR status is incomplete and give a notificationto entry person.

But on PO without PR, the buyer create PO and send for approval process. (we use standart approval hierchy) our espactation like on PR, the status should be incomplete and send a notification. But PO is send by approver, S/He approve it, it's send to other approver, S/He approve it. After last approver, system send a notification to buyer that there is no encumbrance and PO's status wait In Process.