Magento2 – Get Quote Item from Product ID

Find the Extension for free product.

You have to do changes for your requirement.

Make changes in C4B_Freeproduct_Model_Observer function salesruleValidatorProcess

You have to retrieve sku of that product which you want to add into cart as free product (Way you are retrieving in your curent observer) . This function runs for the all products in the cart.

    public function salesruleValidatorProcess(Varien_Event_Observer $observer)
    {

        /* @var $quote Mage_Sales_Model_Quote */
        $quote = $observer->getEvent()->getQuote();

        /* @var $item Mage_Sales_Model_Quote_Item */
        $item = $observer->getEvent()->getItem();

        /* @var $rule Mage_SalesRule_Model_Rule */
        $rule = $observer->getEvent()->getRule();


        /*if ($rule->getSimpleAction() != C4B_Freeproduct_Model_Consts::ADD_GIFT_ACTION
            || $item->getIsFreeProduct()
            || $rule->getIsApplied())
        {
            return;
        }*/

        if ($rule->getSimpleAction() != C4B_Freeproduct_Model_Consts::ADD_GIFT_ACTION
            || $item->getIsFreeProduct())
        {
            return;
        }

          //$item->geQty();

        try {
            //$qty = (int)$rule->getDiscountAmount();
            $qty = $item->geQty();
            //$skus = static::_getSkuList($rule->getGiftSku());
            $skus = static::_getSkuList( /*< Get your sku here for item having Id $item->getId() > */);

            foreach ($skus as $sku) {
                /** @var Mage_Sales_Model_Quote_Item $freeItem */
                $freeItem = static::_getFreeQuoteItem($rule->getId(), $sku, $item->getStoreId(), $qty);
                $quote->addItem($freeItem);
                $freeItem->setApplyingRule($rule);
            }

            //$rule->setData('is_applied', true);
        } catch (RuntimeException $e) {
            Mage::logException($e);
        }
    }

Create one cartrule from admin and select add gift. Add any dummy SKU as we are not using it anywhere.

You can update cart item price using customer login observer. While customer login the cart item will updated with new price.

app/code/Vendor/Customer/etc/frontend/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="customer_login">
        <observer name="vendor_customer_customer_login" instance="Vendor\Customer\Observer\CustomerloginObserver" />
    </event>
</config>

app/code/Vendor/Customer/Observer/CustomerloginObserver.php

namespace Vendor\Customer\Observer;

use Magento\Catalog\Model\Product;
use Magento\Checkout\Model\Cart;
use Magento\Checkout\Model\Session as CheckoutSession;
use Magento\Customer\Model\Address;
use Magento\Customer\Model\Customer;
use Magento\Customer\Model\Session;
use Magento\Framework\Event\ObserverInterface;
use Magento\Quote\Api\CartRepositoryInterface;
use Magento\Quote\Model\Quote\Item;

/**
 * Class CustomerloginObserver
 * 
 */
class CustomerloginObserver implements ObserverInterface {

    /**
     * @var Session
     */
    protected $customerSession;

    /**
     * @var Customer
     */
    protected $customer;

    /**
     * @var Cart
     */
    protected $cart;

    /**
     * @var CheckoutSession
     */
    protected $checkoutSession;

    /**
     * @var Product
     */
    protected $_product;

    /**
     * @var Item
     */
    protected $quoteFactory;

    /**
     * @var CartRepositoryInterface
     */
    protected $quoteRepository;

    /**
     * @var Address
     */
    protected $address;

    /**
     * CustomerloginObserver constructor.
     * @param Session $customerSession
     * @param Customer $customer
     * @param Cart $cart
     * @param CheckoutSession $checkoutSession
     * @param Product $_product
     * @param Item $quoteFactory
     * @param CartRepositoryInterface $quoteRepository
     */
    public function __construct(
        Session $customerSession,
        Customer $customer,
        Cart $cart,
        CheckoutSession $checkoutSession,
        Product $_product,
        Item $quoteFactory,
        CartRepositoryInterface $quoteRepository,
        Address $address
    ) {
        $this->customer = $customer;
        $this->customerSession = $customerSession;
        $this->cart = $cart;
        $this->address = $address;
        $this->checkoutSession = $checkoutSession;
        $this->_product = $_product;
        $this->quoteFactory = $quoteFactory;
        $this->quoteRepository = $quoteRepository;
    }

    /**
     * @param \Magento\Framework\Event\Observer $observer
     */
    public function execute(\Magento\Framework\Event\Observer $observer) {


        $id = $this->customerSession->getCustomerId();
        // check session is active or not.

        if ($session) {

            $itemId = 0;
            $qty = 0;
            $quoteId = 0;
            $sessionItems = $this->checkoutSession->getQuote()->getAllItems();

            foreach ($sessionItems as $session_item) {
                $qty = (int) $session_item['qty'];
                $productId = $session_item['product_id'];
                $product = $this->_product->load($productId);
                $setcustomprice = $this->checkoutSession->getQuote()->getItemById($session_item['item_id']);
                $uom = $product->getUom();

                    if ($price > 0) {
                        $customprice = $this->checkoutSession->getQuote()->getItemById($session_item['item_id']);
                        $customprice->setCustomPrice($price);
                        $customprice->setOriginalCustomPrice($price);
                        $customprice->save();
                        $quoteId = $session_item['quote_id'];
                    }
                }
            }

            if ($quoteId) {
                $quote = $this->quoteRepository->get($quoteId);
                $this->quoteRepository->save($quote->collectTotals());
            }
        }
}