In that instance you honor checks and preauthorized debits for the amount of the partial revocation.
I'm curious how the merchant issuing a refund automatically leads you to conclude that no error occurred.
For example, my card number is compromised and the merchant uses the stolen card information to ship merchandise to a mule they contacted online (e.g. romance scam where the mule is told to receive merchandise on behalf of their online love with instructions to reship the merchandise elsewhere.) Somewhere along the line, the mule figures out they are being scammed and returns the merchandise. The merchant refunds your customer's card less a restocking fee since they are not aware the transaction was fraud.
In this above example, your customer did not authorize the charge and still received a partial refund from the merchant. Under Reg E, an error did occur since the charge was unauthorized and the customer would be entitled to full reimbursement.
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