Transaction inverse - Paypal Express Checkout

La doc est au contraire très claire ;)
il te suffit d'un appel post api avec les infos suivantes

USER=merchant_user_name
PWD=merchant_password
SIGNATURE=merchant_signature
METHOD=RefundTransaction
VERSION=94
TRANSACTIONID=transaction_ID    #ID of the transaction for which the refund is made 
REFUNDTYPE=Full    #Specifies a full refund; a partial refund requires more input fields

J'ai regardé, mon script est donc le suivant :

<?php
/**
 * Send HTTP POST Request
 *
 * @param     string     The API method name
 * @param     string     The POST Message fields in &name=value pair format
 * @return     array     Parsed HTTP Response body
 */
function PPHttpPost($methodName_, $nvpStr_) {
 global $environment;

 // Set up your API credentials, PayPal end point, and API version.
 $API_UserName = urlencode('monid');
 $API_Password = urlencode('monpswd');
 $API_Signature = urlencode('masign');
 $API_Endpoint = "https://api-3t.paypal.com/nvp";
 if("sandbox" === $environment || "beta-sandbox" === $environment) {
 $API_Endpoint = "https://api-3t.$environment.paypal.com/nvp";
 }
 $version = urlencode('51.0');

 // Set the curl parameters.
 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $API_Endpoint);
 curl_setopt($ch, CURLOPT_VERBOSE, 1);

 // Turn off the server and peer verification (TrustManager Concept).
 curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
 curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);

 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 curl_setopt($ch, CURLOPT_POST, 1);

 // Set the API operation, version, and API signature in the request.
 $nvpreq = "METHOD=$methodName_&VERSION=$version&PWD=$API_Password&USER=$API_UserName&
            SIGNATURE=$API_Signature$nvpStr_";

 // Set the request as a POST FIELD for curl.
 curl_setopt($ch, CURLOPT_POSTFIELDS, $nvpreq);

 // Get response from the server.
 $httpResponse = curl_exec($ch);

 if(!$httpResponse) {
 exit("$methodName_ failed: ".curl_error($ch).'('.curl_errno($ch).')');
 }

 // Extract the response details.
 $httpResponseAr = explode("&", $httpResponse);

 $httpParsedResponseAr = array();
 foreach ($httpResponseAr as $i => $value) {
 $tmpAr = explode("=", $value);
 if(sizeof($tmpAr) > 1) {
 $httpParsedResponseAr[$tmpAr[0]] = $tmpAr[1];
 }
 }

 if((0 == sizeof($httpParsedResponseAr)) || !array_key_exists('ACK', $httpParsedResponseAr)) {
 exit("Invalid HTTP Response for POST request($nvpreq) to $API_Endpoint.");
 }

 return $httpParsedResponseAr;
}

// Set request-specific fields.
$transactionID = urlencode('example_transaction_id');
$refundType = urlencode('Full');  // or 'Partial'
$amount;                          // required if Partial.
$memo;                            // required if Partial.
$currencyID = urlencode('USD');   // or other currency ('GBP', 'EUR', 'JPY', 'CAD', 'AUD')

// Add request-specific fields to the request string.
$nvpStr = "&TRANSACTIONID=$transactionID&REFUNDTYPE=$refundType&CURRENCYCODE=$currencyID";

if(isset($memo)) {
 $nvpStr .= "&NOTE=$memo";
}

if(strcasecmp($refundType, 'Partial') == 0) {
 if(!isset($amount)) {
 exit('Partial Refund Amount is not specified.');
 } else {
 $nvpStr = $nvpStr."&AMT=$amount";
 }

 if(!isset($memo)) {
 exit('Partial Refund Memo is not specified.');
 }
}

// Execute the API operation; see the PPHttpPost function above.
$httpParsedResponseAr = PPHttpPost('RefundTransaction', $nvpStr);

if("SUCCESS" == strtoupper($httpParsedResponseAr["ACK"]) ||
   "SUCCESSWITHWARNING" == strtoupper($httpParsedResponseAr["ACK"])) {
 exit('Refund Completed Successfully: '.print_r($httpParsedResponseAr, true));
} else  {
 exit('RefundTransaction failed: ' . print_r($httpParsedResponseAr, true));
}
?>

Et ça me dit que l'authentification est refusée, alors que tout est bon x)

RefundTransaction failed: Array ( [TIMESTAMP] => 2015%2d04%2d14T19%3a17%3a30Z [CORRELATIONID] => 41c235f4452c2 [ACK] => Failure [VERSION] => 51%2e0 [BUILD] => 16204765 [L_ERRORCODE0] => 10002 [L_SHORTMESSAGE0] => Authentication%2fAuthorization%20Failed [L_LONGMESSAGE0] => You%20do%20not%20have%20permissions%20to%20make%20this%20API%20call [L_SEVERITYCODE0] => Error )