Richard J. Leon, a district court judge in Washington, has ruled that the Federal Reserve failed to follow lawmakers' intention of capping fees that banks earn from consumers' use of debit cards. Congress planned to limit the fees that banks earn from retailers and left it to the Fed to set the limits -- which it never did, to the benefit of banks. If Leon's ruling stands, the Fed may have to lower the ceiling on so-called interchange fees, potentially reducing financial firms' revenue from debit cards. Leon said that, in working out the details of the fee cap, the Fed had run “completely afoul of the text, design and purpose” of Congress' intent. The ruling was a response to a lawsuit against the Fed brought by various retailers. Banks and merchants have been in conflict over whether lower debit fees for retailers can benefit consumers. The court has scheduled a hearing for Aug. 14 to decide how to proceed.