1. 1005.6(b) states that liability can be based on a transaction or a series of related transactions. If you conclude the transactions are related (e.g., conducted by the same person, caused by the same data breech, etc.) then you can base your liability calculation on 60 days from the date of the statement on which the 7/26/20 charge appeared. If you conclude they are not related and the 1/6/21 charge would have occurred even if the customer had notified you sooner about the 7/26/20 charge, then they must be treated separately.
2. Although Reg E does not apply, Visa extends its Zero Liability protections to small business debit cards. You have to determine based on your issuer agreement if the customer qualifies for Zero Liability coverage. Unlike Reg E which does not permit you to increase customer liability due to negligence, Visa rules says that Zero Liability may be "withheld, delayed, limited, or rescinded by your issuer based on gross negligence or fraud, a delay in reporting unauthorized use."
A business customer that does not report fraud from July until now certainly had a "delay in reporting unauthorized use" and I would not extend Zero Liability protections to that business.
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