When refunding a payment, the payment gateway uses the original invoice number when the payment was recorded as a connection method for allowing a refund to go through. Since merging an invoice will only keep the one invoice number, if the transaction you're processing occurred on the invoice that no longer exists, you cannot refund the customer within ASAP as the connection is lost between us and the payment gateway. You'll need to refund them directly within the payment gateway and record it in ASAP as an "offline credit card" transaction, leaving detailed notes of what happened. It's also recommended to not merge invoices that involve credit card transactions on each invoice as this situation may come up where you'll be unable to refund your customers.
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