A U.S. appeals court on Monday weighed whether to uphold the approval of an estimated $5.7 billion antitrust settlement by MasterCard Inc and Visa Inc with merchants over credit card fees despite the objection of several major retailers.

A lawyer for retailers including Target Corp and Amazon.com Inc urged the 2nd U.S. Circuit Court of Appeals in New York to reject the deal, saying it forces merchants to give up their rights to sue over various policies and practices.

"The defendants are trying to buy something that is not for sale," said Thomas Goldstein, the lawyer.

But Paul Clement, a lawyer for merchants who negotiated the deal, said the settlement was fair and provided not just $5.7 billion but reforms an expert valued as worth up to $62 billion.

"There was meaningful relief here," he said.

The settlement, the largest in a U.S. antitrust class action, resolved lawsuits by merchants pending since 2005. The lawsuits accused Visa and MasterCard of fixing the fees they were charged each time customers used credit or debit cards.

Merchants also claimed Visa and MasterCard prevented retailers from encouraging customers to use cheaper payment methods.

U.S. District Judge John Gleeson in Brooklyn in 2013 approved the deal, saying it provided significant monetary award and meaningful relief.

The settlement provides for cash payments to merchants and lets them begin charging customers a surcharge when they use Visa or MasterCard credit cards.

Around 8,000 merchants, representing 25 percent of the transactional volume at issue, opted out of the deal, driving its value down to $5.7 billion from $7.2 billion. Over 30 lawsuits by those opt-outs are pending.

Those retailers say a broad litigation release in the settlement forces current and future merchants who accept Visa or MasterCard to forgo rights to sue over various current and future policies and practices.

The deal's approval was thrown into further flux after Keila Ravelo, an ex-lawyer at MasterCard's law firm, was charged in December with engaging in a fraud scheme.

Following her arrest, the firm, Willkie Farr & Gallagher, discovered an exchange of communications between Gary Friedman, a lawyer for the plaintiffs, and Ravelo.

Some retailers say the exchange of confidential information tainted the deal. Similar communications between Friedman and Ravelo resulted in August in a federal judge rejecting a separate antitrust settlement with American Express Co.(AXP.N)

The case is In Re: Payment Card Interchange Fee and Merchant Discount Antitrust Litigation, 2nd U.S. Circuit Court of Appeals, No. 12-4671.

(Reporting by Nate Raymond in New York; Editing by Cynthia Osterman)