Earlier this year, a federal court ruled that American Express violated antitrust laws — resulting in higher costs for consumers — by forbidding retailers that accept AmEx from encouraging customers to use competing cards like those from Visa, MasterCard, and Discover. Now, the city of San Francisco is suing the credit card company to get back billions of dollars for merchants in California.

To quickly recap the issue: merchants often have to a higher per-transaction fee on purchases made with American Express cards than those made through the Visa, MasterCard, or Discover networks. Many retailers want to be able to offer discounts or otherwise reward customers for using those less-expensive cards, but the terms of the AmEx merchants agreement prevents them from doing so.

In a lawsuit that took five years to conclude (and may not be done), the U.S. Dept. of Justice and attorneys general for 17 states claimed that these provisions violated federal antitrust laws.

In Feb. 2015, a U.S. District Court judge in Brooklyn ruled against AmEx, saying that the card company’s contracts resulted in higher prices for consumers.

“By preventing merchants from steering additional charge volume to their least expensive network… the [non-discrimination provisions] short-circuit the ordinary price-setting mechanism in the network services market by removing the competitive ‘reward’ for networks offering merchants a lower price for acceptance services,” explained the court. “The result is an absence of price competition among American Express and its rival networks. In fact, the record shows that merchant prices have risen dramatically in the absence of merchant steering.”

According to San Francisco City Attorney Dennis Herrera, whose office filed the suit [PDF] earlier this month in San Francisco Superior Court, California merchants paid $2.25 billion in merchant fees to AmEx every year. That amount may have been significantly less if these retailers were able to offer some sort of discount for customers who used a competing, less-expensive card or paid cash.

The complaint is asking the court to declare that these clauses in the AmEx merchant contracts violate California law, and to grant an injunction barring the company from enforcing these contractual provisions.

If AmEx is found to have violated California’s Unfair Competition Law, it could face civil penalties of $2,500 for each violation. To Herrera, that doesn’t mean each merchant — which would be substantial given the sheer size of California — but instead applies to each purchase made with an AmEx card in the state.

Finally, the lawsuit seeks unspecified restitution for California merchants affected by the AmEx restrictions.

“The party is over for American Express, and the bill is coming due in California,” Herrera said in a statement. “The federal court ruling earlier this year merely confirms what millions of retailers, economists and U.S. Justice Department officials have known for years: American Express has rigged the game. They shook down merchants, stifled competition, and shifted costs for their extravagant member perks to even cash-paying consumers. It’s unfair, it’s illegal, and — under state law — it warrants tough penalties and restitution for California’s merchants.”

In a statement to the Wall Street Journal, American Express says the San Francisco suit is without merit. The company has already said it intends to fight the earlier District Court ruling.

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