Magento 2 how custom fee refundable

Hi I am using extra fee third party module it working fine. Now I want able to add refund custom fee how can we able to add can an one explain

    <?php

namespace PawanCustomTipModelCreditmemoTotal;

use MagentoSalesModelOrderCreditmemoTotalAbstractTotal;

class Tip extends AbstractTotal
{
    /**
     * @param MagentoSalesModelOrderCreditmemo $creditmemo
     * @return $this
     */
    public function collect(MagentoSalesModelOrderCreditmemo $creditmemo)
    {
         $writer = new ZendLogWriterStream(BP . '/var/log/creditmemotip.log');
        $logger = new ZendLogLogger();
        $logger->addWriter($writer);
        $logger->info('Simple Text Log'); // Simple Text Log
        
        $creditmemo->setTip(0);
        $creditmemo->setBaseTip(0);

        $amount = $creditmemo->getOrder()->getTip();
        $logger->info('Array Log2'.print_r($amount, true)); // Array Log
        $creditmemo->setTip($amount);

        $amount = $creditmemo->getOrder()->getBaseTip();
        $creditmemo->setBaseTip($amount);
        $logger->info('Array Log3'.print_r($amount, true)); // Array Log
        $logger->info('Array Log4'.print_r( $creditmemo->getTip(), true)); // Array Log
        $creditmemo->setGrandTotal($creditmemo->getGrandTotal() + $creditmemo->getTip());
        $creditmemo->setBaseGrandTotal($creditmemo->getBaseGrandTotal() + $creditmemo->getBaseTip());

        return $this;
    }
}