IMPORTANT NOTE:

I just got off the phone with part of the Quickbooks payment/merchant team. In a matter of 30 seconds, they confirmed I was right:

It is not possible to offer a partial refund on a credit or debit card through the Quickbooks Merchant Center.

All of the QB team members above suggesting you can change the amount to reverse the amount is wrong. You can only reverse the entire credit or debit card transaction.

The solution: You can offer a partial refund by creating a brand new refund receipt in QB and processing their credit or debit card, but you are forced to collect their card information again. If you haven't saved their card on file, which is not done by default, then you are unable to provide a partial refund on their card through QB if you do not know their number to type in manually.

It is regrettable that three QB team members shared above incorrect information about the process, including sharing a screenshot where they highlighted an amount field that is uneditable. I twice followed the above instructions in good faith, only to inadvertently reverse the entire transactions. This meant I had to go back to the customer with my tail between my legs, as I thought I was bad at technology. Turns out, I was being given faulty instructions.

To reiterate, you cannot partially reverse a transaction through QB Merchant Center without recollecting their card numbers.