Committing Cost Via Purchase Order

Hi,

I have a two part question.

1. I noticed that when I enter a PO, assign it to a project and the relevant cost code etc. it does not hit my committed cost.  How do I get that PO cost committed?

2. If I get a Prime change approved, then write a PO for some of the material included in that change order, BUT don't generate the PO directly from the approved change order, do I have to commit cost via the Sub/Change tab in the change order or will that create a "double-dip" on the cost side?

Thanks!