Invoice for Serial Number Controlled Items…

Hiii …
How can I select individual serial numbers while we make the Invoice for a serial number controlled items.
Eg. We made a purchase order for 4 quantity (Pcs) of a serial number controlled item.
Serial number created for individual piece while registering that item. Up to this level its ok.
But now I have to post the purchase invoice for 2 piece by selecting 2 serial number.

How can I do it?

Please help me.
Thanks in Advance