DefaultCurrency.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

/**
 * Config Directory currency backend model
 * Allows dispatching before and after events for each controller action
 */
namespace Magento\Config\Model\Config\Backend\Currency;

/**
 * @api
 * @since 100.0.2
 */
class DefaultCurrency extends AbstractCurrency
{
    /**
     * Check default currency is available in installed currencies
     * Check default currency is available in allowed currencies
     *
     * @return $this
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function afterSave()
    {
        if (!in_array($this->getValue(), $this->_getInstalledCurrencies())) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __('Sorry, we haven\'t installed the default display currency you selected.')
            );
        }

        if (!in_array($this->getValue(), $this->_getAllowedCurrencies())) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __('Sorry, the default display currency you selected is not available in allowed currencies.')
            );
        }

        return parent::afterSave();
    }
}