PO Quantity field is editable when it is converted from a PR

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Hi Team,

When we process the requisition and convert the PR into PO, a buyer can go and change the quantity field when the PO is in incomplete status.

This is a control issue for our customer as Buyer can increase the quantity for the favoured supplier and approver might not know, also there might be typo error from buyer side which can cause the issue.

I know there is no standard way to achieve this currently in system, however can we make the field Readonly based on EL expression when the PO is linked with the PR, or can we achieve this using any other work around.