For the purpose of you monetary instrument recordkeeping requirement, since the Cashier's Checks were not purchased in cash you have no obligation.
To minimize the risks associated with the sale of monetary instruments, some institutions have implemented a policy that states that they will not sell monetary instruments to non-customers. They can then be cited for violated their own policy for performing the transactions that you describe. Their AML Risk Assessment may also be called into question if they have not accounted for the risks of sales to non-customers.
You also have to consider the monitoring process for monetary instrument sales and looking for patterns of suspicious activity.
Regardless of whether or not cash is involved, the bank negotiated the check.
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