what happens when you ask the customer for the documentation and they say they don't have anything? don't overthink it. if they say it is unauthorized and provide the written statement of unauthorized debit, take it at face value and move on. perhaps they made other arrangements, perhaps they didn't. you really don't want to get into a process where you review transactions and legal agreements prior to returning or not returning transactions. and, if you do it for one, you better do it for everyone moving forward, or you open the bank to liability and risk.
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Providing alternative truths since the invention of time