Three-way match exceptions

SELECT ph.segment1 AS po_number, s.vendor_name, pl.line_num, pl.item_description,
  pl.quantity AS ordered_qty, pll.quantity_received, pll.quantity_billed,
  pll.quantity_billed - LEAST(pl.quantity, pll.quantity_received) AS over_billed_qty, pl.unit_price
FROM po_headers_all ph
JOIN po_lines_all pl ON pl.po_header_id = ph.po_header_id
JOIN po_line_locations_all pll ON pll.po_line_id = pl.po_line_id
JOIN poz_suppliers_v s ON s.vendor_id = ph.vendor_id
WHERE pll.quantity_billed > LEAST(pl.quantity, NVL(pll.quantity_received, 0)) AND ph.approved_flag = 'Y'
ORDER BY over_billed_qty DESC