You can do that:
echo $row->user_unpaid_amount;
However, that information is only updated on the partner's listing (and on the front end when the user access his partner account). So it's quite useless on the orders listing by itself. That's why I said that you would first need to load that data from MySQL but that's really not easy to do as there are several queries to use for that, like we did in the listing function of the file administrator/components/com_hikashop/views/user/view.html.php for the partners listing page.