Judge Leon was referring to the Fed's implementation of the Durbin Amendment of the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010. Durbin requires caps on the interchange fees charged by banks to process debit card purchase transactions for merchants. The Durbin Amendment also requires greater choice for merchants on which transaction network to use. The judge believes the spirit of the Durbin Amendment was to require merchants to have a choice of payment network for each debit card transaction, rather than for each debit card. If Judge Leon's ruling survives the announced appeal by the Federal Reserve, it will cause a huge changeover by banks and merchants, because some debit cards are only coded for use on one network. With a roughly 71% market share for debit card purchases, Visa has the most at stake from Judge Leon's ruling. Visa's shares have dropped 8% since July 31, while MasterCard's shares have risen 2%. Visa's underperformance following Judge Leon's ruling prompted KBW analyst Sanjay Sakhrani on Aug. 14 to call the event a " buying opportunity" for investors. "We believe the Fed appeal can be won," Wells Fargo analyst Timothy Willi wrote in a note to investors on Friday, adding that the regulator "should be well prepared to defend their decisions," on how to interpret the language of the Dodd-Frank legislation.