Investors were pleased with Mastercard's second-quarter results, reported late on Wednesday. The payment processor reported second-quarter earnings of $848 million, or $6.96 a share, increasing from $700 million, or $5.56 a share, in the second quarter of 2012, excluding special litigation charges taken during the prior period. Net revenue was up 15% year-over-year, to $2.096 billion. Mastercard said the revenue growth was driven by a 13% increase in transaction processing volume, to $1 trillion, with an 11% increase in the number of processed transactions, to 9.5 billion during the second quarter. Oppenheimer analyst Glenn Greene rates Mastercard "outperform," with a 12-to-18-month price target of $640, and in a note to clients on Wednesday wrote that the stellar second-quarter results were "driven by solid revenue/volume growth, lower incentives, and 400bps of margin expansion." "However, overshadowing the strong results, a US district court judge opined against the Federal Reserve's interpretation of the Durbin amendment, thereby shifting sentiment, and creating a new regulatory overhang for Visa and MasterCard," Greene wrote. Greene was referring to a ruling on Wednesday by U.S. district court judge Richard J. Leon in Washington that the Federal Reserve had "clearly disregarded Congress's statutory intent" when it finalized rules to implement the Durbin Amendment in 2011. The Durbin Amendment -- named after Senator Richard Durbin (D., Ill.) -- of the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010 placed limits on the interchange fees paid by merchants to banks to processes debit card purchase transactions. The amendment also gave retailers more freedom to choose which network to use to route debit card payments. Durbin covered banks with total assets exceeding $10 billion. The Federal Reserve in December 2010 estimated that the average debit card interchange fee paid during 2009 was 44 cents, and initially proposed a cap on interchange fees at 12 cents. After the usual comment period, the Fed in June 2011 issued its final ruling, for an interchange fee cap of 21 cents, plus an additional "5 basis points multiplied by the value of the transaction." A group of retailers sued the Federal Reserve, seeking to overturn the new interchange rules, arguing that the Fed had taken "other costs" not "associated with authorization, clearing, and settlement ('ACS') of an electronic debit transaction," when setting the basic 21-cent interchange fee cap. The plantiffs also argued that the Durbin amendment called for merchants to have multiple choices of unaffiliated payment networks for each debit card transaction, and not just for transactions requiring the consumer to enter a PIN code.