If you are using Authorize.net for your Payment Gateway, you will not be able to refund your customer if the transaction is more than four months old. However, you can still "credit" the credit card by following these steps below.
In order to "credit" or put funds onto a card, you must enable "Expanded Credit-Return Capabilities" through Authorize.net.
Click here for Authorize.net's answer to: What are expanded credit capabilities?
There is no additional charge for this, but you must fill out a one page application. The reason this is not enabled by default is because they do not want someone to be able to maliciously credit a card. In the application, they ask for the length of time this functionality should be enabled for. You can determine whether you want this functionality enabled for just a limited time period or indefinitely.
Once enabled, you can click on 'New Payment', select 'Credit Card', choose the credit card, and then put in a NEGATIVE number to credit the card with that amount.
You can see if this is enabled for your account by navigating to your Merchant Profile in Authorize.net
Login to your Authorize.net account → ACCOUNT → Merchant Profile
In your Merchant Profile check the status look for "Expanded Credit Capabilities (ECC)" listed under Additional Services.
The application can be found below. The top of the application tells you how to submit it through the "Merchant Interface".
Please expect Authorize.net to take 3 business days for them to review the form.
Another option if you don't want to enable Expanded Credit-Return Capabilities would be to mail the customer a check.
Please feel free to reach out to us if you have any questions about this.
Authorize.net's 24/7 phone support: (877) 447-3938