Guarantee that goods are delivered before payment

Hi,

Currently we are working on a project to develop a solution that supports the internal purchase process. The idea is: the employee creates a purchase requisition in ME51N; it goes through a customized workflow; if it is validated and approved, the PR will then be automatically converted in a specific type of PO (that we've created to distinguish them from others; let's call it XYZ). Since the PR goes through an extensive workflow of validation and approval, when they're converted in XYS PO, the PO is instantly blocked and released.

To make sure that we don't pay the goods before they arrive, by default, the "Goods Receipt" checkbox on the Delivery tab of items is checked. When the goods arrive, the creator of the PR that originated this PO is supposed to go to the line of the item received and uncheck the "Goods Receipt" checkbox. The problem is that the PO is blocked (so that no one can change the number of units purchased, for example) and so they will not be able to change this checkbox.

Is there a way to block every field except this checkbox? Or is there a way to block the payment until the goods are received? If so, what should we do to indicate that the goods have been received?
(We want the solution to be simple for the PR creator. Currently someone has to uncheck this box on PO so that they can register the invoice. If the solution is "complex" the PR creator could just ignore this procedure, and move on after receiving the goods he/she ordered).

Thanks in advance!