As your statement..
We can use the 500 remaining pcs for PO.
There are no configuration about it.
Can you check the PR status
What is the PR quantity and What it shown in Status tab?
See the status
My PR content 1500 pc, I have made 500 BOX of PO (1 BOX = 10 Pcs)
It means my PR order quantity is full.
What about yours?