Undelivered PO...what happens to Req?

Hi,

I wanted to confirm something.

If I have a Requisition for 10, I convert this PO to 10 but the vendor only delivers 8 and he will not be delivering any more for whatever reason. Am i right in assuming if i click on the delivery indicator in the PO that should be it?

I have been asked questions by users saying we should be changing the quantity of the PO to 8 then changing the req quantity etc.. Should we?

Thanks