How to get the form “Add to cart” programmatically through an AJAX callback

I use this code to get the form with button 'Add to cart'.

$price_arr = commerce_product_calculate_sell_price($product);
$price = commerce_currency_format($price_arr['amount'], $price_arr['currency_code'], $product);

$cart_form_id = commerce_cart_add_to_cart_form_id(array($product->product_id));
$line_item = commerce_product_line_item_new($product);
$line_item->data['context']['product_ids'] = array($product->product_id);

$cart_form_arr = drupal_get_form($cart_form_id, $line_item);
$cart_form_arr['#action'] = '/' . $lang . '/gallery';

$cart_form_html = drupal_render($cart_form_arr);

When I do

return $cart_form_html;

there is no problem. This code works in the back-end.

I have an ajax callback (a Drupal menu link) - '/get-product-button/ajax/[productID]'. This callback returns a value through

drupal_json_output($cart_form_html);

(the same variable as in the back-end) which I print using JS.

This returned value contains an HTML code with a button, but it contains another form token, and this code does not work (it does not add to cart anything, but reloads the page).

Any idea?