Error with Parse.com using Javascript API: “Parse::UserCannotBeAlteredWithoutSessionError”

I get an "Parse::UserCannotBeAlteredWithoutSessionError" error using Parse in AngularJS when I use a copied user to modify data. The operations are done well, but the browser console shows the error. Here is the code related:

function createEmployeeCtrl($scope, $rootScope) {

    $scope.loggedUser = Parse.User.current();
    // Deep copy
    var userCopy = jQuery.extend(true, {}, $scope.loggedUser);

    // All data will be store in this object
    $scope.formData = {};

    var profileType = $scope.loggedUser.get("profileType");

    $scope.usageRemainingLimit = $scope.loggedUser.get("usageRemaining");

    // After process wizard
    $scope.processForm = function() {
       //INIT PARSE
        var user = new Parse.User();
       //... more code
        var uso = 0;
        var sessionToken = $scope.loggedUser._sessionToken; // I save current user token

        user.signUp(null, {
            success:function(user) {
                Parse.User.become(sessionToken).then(function(user) {
                    Parse.User._saveCurrentUser(user);
                }, function (error) {
                    alert("user not set");
                });
                userCopy.set("usageRemaining", $scope.usageRemainingLimit - uso);
                userCopy.save();
                //...window.location.href to a view of my app
            },
            error:function(user, error) {
                console.log("ERROR!");
                alert("Error: " + error.code + " " + error.message);
            }
        });
    };
}