Magento partial refund creditmemo programatically

Mage::app('admin'); //You can create creditmemo in admin area only

//You should have $orderID as order increment ID and $sku as product SKU you want to refund for.

$order = Mage::getModel('sales/order')->loadByIncrementId($orderID);

$orderItem = $order->getItemsCollection()->getItemByColumnValue('sku', $sku);

$service = Mage::getModel('sales/service_order', $order);

$data = array(

    'qtys' => array(

        $orderItem->getId() => 1 //qty to refund.. $orderItem->getQty()

    )

);

$creditMemo = $service->prepareCreditmemo($data)->save();