Magento : How to place an order for free ( using free payment option) in admin?

Some time we need to place and order for our customer as a demo or any missing products in last orders, in such of conditions we create order manually, and we don’t need to use any payment method to create such types of orders.
In that situation we need a free payment option in admin and not for front end. I am going to share with you how we can use free payment option in admin which is shows in follwoing steps:

1. Go to app/code/core/Mage/Payment/Model/Method and open file Free.php file.

2. Then go to Line 48-49 you will notice following code:

 protected $_canAuthorize                = true;
 protected $_canCapture                  = true;

3. After these lines add 2 lines of custom code as show in following:

 protected $_canAuthorize                = true;
 protected $_canCapture                  = true;
 protected $_canUseInternal              = true;
 protected $_canUseCheckout              = false; 

4. Next you will notice follwoing code :

public function isAvailable($quote = null)
    {
        return parent::isAvailable($quote) && (!empty($quote))
            && (Mage::app()->getStore()->roundPrice($quote->getGrandTotal()) == 0);
    }

5. Replace above code with the following code (in this function only change ‘== 0′ to ‘> 0′):

public function isAvailable($quote = null)
    {
        return parent::isAvailable($quote) && (!empty($quote))
            && (Mage::app()->getStore()->roundPrice($quote->getGrandTotal()) > 0);
    }

6. Now save Free.php file and upload it to custom local location app/code/local/Mage/Payment/Model/Method/Free.php

7. Next go to Admin panel and using navigation open System -> Configuration -> Payment Methods then Enable Zero Subtotal Checkout payment method, you can rename its title according to your desire.

8. Now clear Magento cache.

9. Done, now when to create any new order from admin you will see Zero Subtotal Checkout. it will not show on front end.

10. Hope it help you

11. For more detail visit Magento Post.