Purchase order restrict finally closed status when invoice is pending thru personalisation

HI ,

We have a requirement where we need to block all the  users to "Finally Close" the Purchase orders where Goods Receipted but Invoice not booked It should allow only where ordered qty and received qty and billed qty is matching.

Here i am planning to trigger an Event : When the user choosing Finally Close Option for Purchase order it should validate and throw an error if the lines from the PO received but not invoiced.

Please help

Ravi.G