src/Payum/Paypal/ExpressCheckout/Nvp/Api.php
public function doReferenceTransaction(array $fields)
    {
        $fields['METHOD'] = 'DoReferenceTransaction';

        $this->addVersionField($fields);
        $this->addAuthorizeFields($fields);

        return $this->doRequest($fields);
    }
public function doReferenceTransaction(array $fields)
    {
        $fields['METHOD'] = 'DoReferenceTransaction';

        $this->addVersionField($fields);
        $this->addAuthorizeFields($fields);

        return $this->doRequest($fields);
    }
Require: AMT, PAYMENTACTION, REFERENCEID

@param array $fields

@return array