Pasarela Sermepa/Redsys Cobro de cantidades pendientes

Si se ha realizado un pago inicial con la pasarela usando la opción de dejar pendiente X€ para cobrar posteriormente, para cobrarlo se realizará de la siguiente forma:

$name = 'AXX XXXX';
$code = 'XXXX'; //codi del comerç
$terminal = '1';
$currency = '978';
$transactionType = '6';
$MerchantData = "1";

$dataMaximaCobramentDiferit = '2013-10-15';
$order = $_REQUEST['order_id'];

$urlMerchant = 'http://www.ultracavallsdelvent.com/privat/scripts/retorns-cobraments/cobrament-diferit.php';

$urlOK = 'http://www.ultracavallsdelvent.com/public/scripts/sorteo/ok.html';
$urlKO = 'http://www.ultracavallsdelvent.com/public/scripts/sorteo/ko.html';

$titular = '';
$producto = 'Cobrament Inscripció';

//Dades necessàries per al retorn
$amount = $preu * 100;//Preu de la preinscripcio
$freqRecursiva = 1;
$firma = $amount . $order . $code . $currency . $transactionType . $urlMerchant . $clave;
$signature = strtoupper(sha1($firma));

<form id="returnForm" action="<?php echo $url_tpvv;?>" method="post">
        <table>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_Amount" value="<?php echo $amount;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_Currency" value="<?php echo $currency;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_Order" id="orderNum" value="<?=$order?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_ProductDescription" value="<?php echo $producto;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_Titular" value="<?php echo $titular;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_MerchantCode" value="<?php echo $code;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_MerchantURL" value="<?php echo $urlMerchant;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_UrlOK" value="<?php echo $urlOK;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_UrlKO" value="<?php echo $urlKO;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_MerchantSignature" id="signatura" value="<?=$signature?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_TransactionType" value="<?php echo $transactionType;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_MerchantData" value="<?php echo $MerchantData;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_DateFrecuency" value="<?php echo $freqRecursiva;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_ChargeExpiryDate" value="<?php echo $dataMaxima;?>">
                </td>
            </tr>
            <tr>
                <td>
                    <input type="hidden" name="Ds_Merchant_Terminal" value="<?php echo $terminal;?>">
                </td>
            </tr>            
        </table>
    </form>