$(document).on('DOMNodeInserted', function (e) {
   if ($('.new')) {
      $('.but').attr("disabled", true);
      alert('!!!');
   }
});

$(document).on('DOMNodeRemoved', function (e) {
   var $element = $(e.target).find('.new');
   if (!$element.length) {
      $('.but').attr("disabled", false);
      alert("111");
   }
});