That is wrong if AHLI UNITED BANK didn't have ur written consent.
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I think you must have acted as a GURANTOR on his behalf so he could get a high limit or atleast be eligible to have CREDIT CARD. That's why, whenever his salary/deposit was over due, they used to debit ur account for the same outstanding MONTHLY PAYMENT.
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This is stupidity that they are asking you to get letter from ur friend asking the bank to stop debiting ur account. This doesn't make any sense as per my thinking as a BANKER

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