Order::createPayment

static function createPayment(BitrixSaleOrder &$order, $paySystemId = 0)
{
	$payments = $order->getPaymentCollection();

	/** @var BitrixSalePayment $payment */
	if($payments->count() > 0)
	{
		foreach ($payments as $payment)
		{
			if($payment->isPaid())
				$payment->setPaid("N");

			$payment->delete();
		}
	}

	$payment = $payments->createItem();

	if(intval($paySystemId) > 0)
	{
		$psName = 'Not found ('.$paySystemId.')';

		if($ps = PaySystemManager::getById($paySystemId))
			$psName = $ps['NAME'];

		$payment->setField('PAY_SYSTEM_ID', $paySystemId);
		$payment->setField('PAY_SYSTEM_NAME', $psName);
	}

	$payment->setField("SUM", $order->getPrice());

	return $payment;
}