This is for stores using currencies which is not USD. then follow the steps below
modify the catalog/ext/modules/payment/paypal/express.php file.
Find:
$params = array();
$params['AMT'] = $paypal_express->format_raw($order->info['total'], ”, 1);
Replace with:
$params = array(‘CURRENCYCODE’ => $order->info['currency']); $params['AMT'] = $paypal_express->format_raw($order->info['total']);
Then discount code works perfect for currencies like GBP, AUD etc which is not the default USD