private void Purchasing_Selected(object sender, RoutedEventArgs e)
        {
            transaction purchase = new transaction();

            stack.Children.Clear();
            stack.Children.Add(purchase);
        }
        private void close_transaction()
        {
            transaction purchase = new transaction();

            foreach (Window window in Application.Current.Windows)
            {
                if (window.GetType() == typeof(MainWindow))
                {
                    (window as MainWindow).stack.Children.Clear();
                    (window as MainWindow).stack.Children.Add(purchase);
                }
            }
        }