Выводить кнопку оплаты через API

$orderObj = Sale\Order::load($order['ORDER']['ID']);

$paymentCollection = $orderObj->getPaymentCollection();

$payment = $paymentCollection[0];

$service = Sale\PaySystem\Manager::getObjectById($payment->getPaymentSystemId());

$context = \Bitrix\Main\Application::getInstance()->getContext();

$service->initiatePay($payment, $context->getRequest());