I am helping out in Customer Service and accidentally refunded a customer for their full order instead of just the one item they returned. I went into First Data and VOIDED the transaction but now we still have the entire order returned in Magento. What do I need to do?
You did voided transaction ONLINE which means at the gateway. Because its First Data and if custom extension was used for implementation of the gateway it wont be visible in Magento.
However in some cases in magento when you open Credit Memo there is cancel button. You can try to check if it there.
I hope this should work, i don't see a cancel button but perhaps i will talk to our payment gateway support to see if the request for full refund was honored, if not then please refund the partial amount talking to payment provider offline.