The current logic will not allow a user to change the PO quantity if there is an open Put-Away for the item against the PO. Not allowing with an open warehosue receipt makes sense but if there is no unposted whse receipt and only a put away the user should be able to change the PO.
As long as you are changing the quantity to an amount greater than or equal to the tot al quantity received and there in not an unposted warehosue receipt for the item allow the user to change.
This is a simple change to just change the document types you are checking for when you validate a quantity change.