PR should not change after PO Creation without Unrelease

Hi,

We want that User cann't change the PR after PO Creation.

For Ex.:- We create a PR with 10 Qty., release it and also create PO with 10 Qty.. Now after PO creation with same qty, no one can change PR (Qty, Purchase Group etc...)..

If User Un-release the PO and PR, then user should be able to change in PR...

Please guide...