To solve your issue, you can convert your 'qty' field value to a string value, or override the 'getValue' method of numberbox.
<script type="text/javascript">
(function($){
    $.extend($.fn.numberbox.methods, {
        getValue: function(jq){
            value = jq.textbox('getValue');
            return parseFloat(value)||'';
        }
    });
})(jQuery);
</script>