Magento – No credit memo button available if the end price if 0

It is not possible to create a credit memo for an order that does not exist. It is a problem I have had to face in the past when I have migrated a solution from one platform to another. In some cases I have imported an amount of previous orders into magento so that the merchant can deal with such situations but for a single event as you describe it would take more time than creating one my had.

The reason for not being able to create a credit memo without an order is down to the way that data is stored within Magento. Orders have a relationship / history to payment, customer address etc. A credit memo would require at minimum a order entity ID that will not exist within your system so there would be no integrity for the data. No way to view load edit save etc.

Try this :

<?php
require_once('app/Mage.php');
Mage::app('default');

class creditmemo
{
    public function index()    
    {
        $creditmemoData = array(
        'qtys' => array('order_item_id' => 3, 'qty' => '1'),
        'shipping_amount' => null,
        'adjustment_positive' => '0',
        'adjustment_negative' => null
    );

     $comment= 'comment for credit memo';

     $notifyCustomer =  true;
     $includeComment = false;
     $refundToStoreCreditAmount= '1';            
        $order = Mage::getModel('sales/order')->load(101);
        //OR $order = Mage::getModel('sales/order')->loadByIncrementId('100001');

        // Case : 1 partial credit memo

            /*$orderItem = $order->getItemsCollection()->getItemByColumnValue('sku', 'test-1');
            $creditmemoData = array(
                'qtys' => array( $orderItem->getId() => 1)
            );*/


        // Case : 2 full credit memo 
        // $creditmemoData = array();  

        /** @var $service Mage_Sales_Model_Service_Order */
        $service = Mage::getModel('sales/service_order', $order);
        /** @var $creditmemo Mage_Sales_Model_Order_Creditmemo */
        $creditmemo = $service->prepareCreditmemo($creditmemoData);

        $creditmemo->setPaymentRefundDisallowed(true)->register();
        // add comment to creditmemo
        if (!empty($comment)) {
            $creditmemo->addComment($comment, $notifyCustomer);
        }
        try {
            Mage::getModel('core/resource_transaction')
                ->addObject($creditmemo)
                ->addObject($order)
                ->save();
            // send email notification
            $creditmemo->sendEmail($notifyCustomer, ($includeComment ? $comment : ''));
        } catch (Mage_Core_Exception $e) {
             $e->getMessage();
        }
        return $creditmemo->getIncrementId();
    }
}
$obj = new creditmemo();
$obj->index();
?>