Banks have long argued that processing debit card transactions by starting with the largest dollar balances and working their way to the smallest benefited customers. Here's an alternate explanation of what might have motivated them: "Multiple Millions $$$$$$$$$$$$$$$$$$$$."

That was the pitch a popular vendor of "high-to-low" debit transaction processing software delivered to Union Bank of California, a unit of Mitsubishi UFJ Financial Group, according to documents recently unsealed in a consolidated federal class action.

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