Devolción Parcial

    <?php
    ini_set("display_errors", 1);
    $end_point = "https://sandbox.decidir.com/services/t/1.1/Authorize.AuthorizeHttpSoap12Endpoint";
    $wsdl = "https://sandbox.decidir.com/services/t/1.1/Authorize?wsdl";

    $http_header = 'Authorization: PRISMA XXXXXXXXXXXXXXXXXXXXXXXX';

    $headers = array(
      'http'=>array(
        'header'=>$http_header)
    );

    $contexto = stream_context_create($headers);

    $array_de_datos = array(
        "Merchant"=>"12345678",
        "EncodingMethod"=>"XML",
        "Security"=>"PRISMA XXXXXXXXXXXXXXXXXXXXXXXX",
        "Operation"=>"Devolución",
        "Payload"=>"
        <Request>
        <NROCOMERCIO>12345678</NROCOMERCIO>
        <NROOPERACION>9238742989</NROOPERACION>
        <MONTO>10</MONTO>
        </Request>"
        );

    $soap_client = new SoapClient($wsdl, array(
        'stream_context'=>$contexto,
        'location'=>$end_point,    
        ));

    $rta = $soap_client->Execute($array_de_datos);
    echo "<br />";
    print_r($rta);

Respuesta:

    stdClass Object ( 
        [StatusCode] => -1 
        [StatusMessage] => Devolución aceptada
        [AuthorizationKey] => A2B770F888CF4FEE7B1441914555812 
        [EncodingMethod] => XML 
        [Payload] => stdClass Object ( 
            [Answer] => stdClass Object ( 
                [IDMOTIVO] => 0 
                [RESULTADO] => APROBADA 
                [FECHAHORA] => 2015/09/10 16:49:14 
            ) 
        ) 
    )