You're correct in that VFP doesn't support sub-queries of this type. (I think VFP 9 will, but that won't be out for a while.) Here's how you need to do this:

SELECT h.pid_owner, h.ord_cod, ;
       SUM(d.itm_prcus * d.qty_ord) as OrderTotal, ;
       SUM(i.itm_prcus * i.qty_sent) as InvoiceTotal ;
   FROM C:\QN\datatmp\qn_head h ;
      INNER JOIN C:\qn\datatemp\qn_dtl d ;
         ON d.pid_owner = h.pid_owner AND d.ord_cod = h.ord_code ;
      INNER JOIN c:\qn\datatemp\qn_inv i ;
         ON i.pid_owner = h.pid_owner AND i.ord_code = h.ord_cod ;
   ORDER BY h.pid_owner, h.ord_cod