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>