ProductManager::getPayableQuantityByBasketItem

protected function getPayableQuantityByBasketItem(BasketItem $item): float
{
	$quantity = 0;

	if ($this->order)
	{
		/** @var Payment $payment */
		foreach ($this->order->getPaymentCollection() as $payment)
		{
			/** @var PayableBasketItem $payable */
			foreach ($payment->getPayableItemCollection()->getBasketItems() as $payable)
			{
				$basketItem = $payable->getEntityObject();
				if ($basketItem->getBasketCode() === $item->getBasketCode())
				{
					$quantity += $payable->getQuantity();
				}
			}
		}
	}

	return $quantity;
}