That amount is calculated by the form, its not saved in a field because it changes as more invoices are matched.
You can get it with a query like this:
select poh.segment1 po_Number, sum(aid.amount) matched_amount
from po_headers_all poh, po_distributions_all pod, ap_invoice_Distributions_all aid
where aid.po_distribution_id = pod.po_distribution_id
and pod.po_header_id = poh.po_header_id
group by poh.segment1
That will give you the sum of the ap invoice amounts matched to distributions on the PO number. If you need the po line or shipment added, just let me know and I can show you how to get those as well.
Hope this isn't too late to be helpful,