From NewHaven Software Wiki

Overview

Authorize.net offers two methods of processing credits (refunds), Linked and Unlinked, and there are important differences. The most important of these is CMS currently only supports Unlinked Credits. This article explains the differences between them and includes useful related links and information.

Unlinked Credits

This transaction type is used to issue a refund for a transaction that was not originally submitted through the payment gateway. It also allows the merchant to override restrictions for submitting refunds for payment gateway transactions, for example, if the merchant is beyond the 120-day period for submitting a refund or would like to refund an amount that is greater than the original transaction amount.

The ability to submit unlinked credits is not a standard feature of a merchant’s payment gateway account. To be enabled for expanded credits capability (ECC), the merchant must submit an application. For more information about the ECC application please click the ECC application link.

CMS processes all refunds through Authorize.net as unlinked credits. As such you must enable the ECC option in your account by completing and submitting the ECC application.