The Problem:

You’ve enabled partial refunds in the Authorizenet Model, but it’s not refunding the proper amounts.  This is due to, in my view, a bug in the core which I’ll show you how to patch!

The Solution:

What you need to do is overload the refund method in the Authorizenet model.

Create a new model that extends the original Authorizenet model.  You’ll need to follow the general instructions for how to create a custom module if you don’t already know how to do so.

class Spinonesolutions_General_Model_Paygate_Authorizenet extends Mage_Paygate_Model_Authorizenet {
//Override Mage_Paygate_Model_Authorizenet

protected $_canRefund = true;

public function refund(Varien_Object $payment, $amount) {
  $error = false;
  if ($payment->getRefundTransactionId() && $amount>0) {
    $payment->setAnetTransType(self::REQUEST_TYPE_CREDIT);
    $request = $this->_buildRequest($payment, $amount);
    $request->setXTransId($payment->getRefundTransactionId());
    $result = $this->_postRequest($request);
    if ($result->getResponseCode()==self::RESPONSE_CODE_APPROVED) {
      $payment->setStatus(self::STATUS_SUCCESS);
    } else {
      $error = $result->getResponseReasonText();
    }
  } else {
    $error = Mage::helper('paygate')->__('Error in refunding the payment');
  }

  if ($error !== false) {
    Mage::throwException($error);
  }

  return $this;
}
}

Now all we need to do is let Magento know that we’ve got a custom model.  In config.xml add

<?xml version="1.0"?>
<config>
  <modules>
    <Spinonesolutions_General>
      <version>0.0.1</version>
    </Spinonesolutions_General>
  </modules>
  <global>
    <models>
      ...
      <paygate>
        <rewrite>
          <authorizenet>Imagistic_General_Model_Paygate_Authorizenet</authorizenet>
        </rewrite>
      </paygate>
      ...
    </models>
  </global>
</config>

That should do it!  Questions and comments welcome.

Magento, Model