src/Message/ExpressCompletePurchaseResponse.php
public function getTransactionReference()
    {
        if ($this->isSuccessful() && isset($this->data['PAYMENTINFO_0_TRANSACTIONID'])) {
            return $this->data['PAYMENTINFO_0_TRANSACTIONID'];
        }

        return parent::getTransactionReference();
    }
public function getTransactionReference()
    {
        if ($this->isSuccessful() && isset($this->data['PAYMENTINFO_0_TRANSACTIONID'])) {
            return $this->data['PAYMENTINFO_0_TRANSACTIONID'];
        }

        return parent::getTransactionReference();
    }
The transaction reference obtained from the purchase() call can't be used to refund a purchase.

@return string