Pass Variable From JQuery To PHP via Link

Have a link that passes the pid value through a URL to go to JQuery, the link is shown below:

<td class="listingTextLeft">
<a href="?pmntid=<?php echo $row[0] ?>" data-pid="<?php echo $row[0] ?>" class="pmntDetail"><?php echo $row[20] ?>
</a></td>

this passes the pid through to the JQuery below:

jQuery('.pmntDetail').each(function(i,v){
    jQuery(v).click(function(paymentID){
     paymentID.preventDefault();
     paymentID.stopPropagation();
     var pmntid = $(this).data("pid");
     console.log("ID: ", pmntid);
        $.ajax({
            type: 'post',
            url: 'payments.php',
            data: { idNumber: pmntid },
                success: function(data) {
                    $("#pmntDetailPopup").modal({position: ["5%"]});    
                }
        });
  });
});

The console log shows the correct value for the pid and idNumber is shown in Post in Firebug correctly, however I am unable to pass it through to a popup div on the same page to be used in another query, echo $_POST['idNumber'] gives a blank.

Have read other answers on here and other sites but have been unable to get them to work.

What I would like to do is pass the pid variable to the SQL query that is used to populate the jquery modal popup box.

**EDIT ** Have updated the JQuery code with the ajax added, is sitll not passing the variable through.