The Duke Semans mansion on Fifth Avenue was sold for $44 million to Carlos Slim Helú.

The sale in a down market was eye-popping: $44 million, in July, for a town house that had sold for $40 million in 2006.

It was the city’s biggest residential sale of 2010, and the fourth-highest price on record for a town house, according to city property data. The backstory has a winding plot worthy of an HBO or Showtime series.

The stars are the Duke tobacco family’s Fifth Avenue mansion; an immigrant cabdriver turned Fortune 500 real estate mogul; a Mexican billionaire; and one of the city’s most powerful brokers. The broker’s firm sued the mogul for making a secret deal to sell the mansion to the billionaire, accusing him of breaching an exclusivity agreement and not paying out nearly $1 million in commission.

It all started with Tamir Sapir, who immigrated to the United States 30 years ago from Georgia in the former Soviet Union and worked as a cabdriver before borrowing $10,000 against his taxi medallion to start an electronics business. What followed was something like the American dream on steroids, as Mr. Sapir, who went on to make his fortune in Russian oil and become a Manhattan real estate tycoon, made a splash with his purchase of the $40 million town house, known as the Duke Semans mansion.

Real Estate Highlights

In this week’s section: Demand for apartments with washers and dryers is growing; a Brooklyn couple moving in together searches for an apartment where they can spend time apart; a new disclosure law for real estate brokers.

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When he bought the seven-floor, roughly 20,000-square-foot town house at 1009 Fifth Avenue, a cadre of yellow cabs circled the mansion to celebrate. Across from the Metropolitan Museum of Art, it is a Beaux-Arts behemoth, one of the few remaining single-family residences in an area once known as Millionaire’s Row.

The Duke family had hired Paula Del Nunzio, one of the city’s highest-selling brokers, and Shirley A. Mueller, both of Brown Harris Stevens, to handle the 2006 sale, along with Sharon E. Baum of the Corcoran Group. The Brown Harris Stevens brokers represented Mr. Tapir in 2010 when he put the mansion on the market for $50 million.

But according to a lawsuit filed last August by Brown Harris Stevens, Mr. Tapir signed a $44 million contract with Carlos Slim Helú — whom some have described as the world’s richest man — after he had already accepted a $37 million offer from another buyer secured by the Brown Harris Stevens brokers.

According to the suit, filed in State Supreme Court in Manhattan, Mr. Tapir breached his exclusivity agreement with Brown Harris Stevens by striking the deal with Mr. Slim on the side. The negotiations between the mogul and the billionaire were conducted by Soly Halabi, the senior managing director of Venture Capital Properties, who is relatively new to the business but has worked on other deals for Mr. Slim.

Broker commissions are typically split 50-50 between the representatives of the buyers and the sellers, but Brown Harris Stevens did not receive its share of the commission, or $880,000, according to the complaint.

(Mr. Slim is a creditor and minority shareholder of The New York Times Company.)

The dispute centered on whether the seller’s exclusive agreement with Brown Harris Stevens was in effect when he made the deal with Mr. Slim.

The parties reached a settlement in December for an undisclosed amount. Ms. Del Nunzio and Hall F. Willkie, the president of Brown Harris Stevens, said in interviews that it had included the brokers’ compensation for the commission. “There was a disagreement about what the roles were,” Mr. Willkie said. “But the owner agreed we deserved our compensation.”

Calls to Mr. Sapir’s offices at the Sapir Organization in Manhattan were not returned.

Mr. Halabi, the rookie broker, said that even though he and Venture Capital Properties had been named as defendants in the lawsuit, the settlement cleared them of any wrongdoing and of responsibility for any payment to Brown Harris Stevens, although there is no corroboration of that because the settlement was not made public.

Josh Rahmani, the chief executive of Venture Capital Properties, said, “As far as the deal goes, it was a real opportunity for the purchaser. There’s only one Duke Semans mansion.”

So true. But whose deal was it?

Ms. Del Nunzio said the lawsuit had overshadowed what was a stunning real estate victory, no matter who wanted to claim it. “The important thing is that the sale occurred,” she said. “A $44 million sale occurred in these current market conditions. That is utterly phenomenal.”

As for the legal dispute, she said, “The situation has been resolved, and we’re really happy about that.”

‘Architecturally Quiet’ in the Far West Village

Annabelle Selldorf, in 2004, is designing a residential building in the Far West Village.

The architect Annabelle Selldorf has drawn a lot of attention for high-profile projects like the shimmering high-rise condominium at 200 11th Avenue in West Chelsea, the lobby and interiors of the Urban Glass House in SoHo, and studios for artists like Jeff Koons.

But her residential projects also include some lower-key endeavors, listed on her firm’s Web site as, simply, “East Village town house,” “SoHo loft” and “Long Island residence.”

Another Selldorf creation hasn’t made that list yet, but it will rise at 335 West 12th Street (or 802 Greenwich Street; the final address has not been decided), where the city has approved construction of a four-story building.

A spokeswoman for Ms. Selldorf declined to disclose the kind of details that delight Selldorf fans in the architecture world, who often describe her Modernist work as having an elegant restraint.

But Karen Thomas, the president of Karen Thomas Associates, the project manager, said the 11,000-square-foot property would be “architecturally quiet,” like most of the Far West Village. The building was approved as a one-, two-, or three-family residence with commercial space on the ground floor. Ms. Thomas would not say how the residence would be configured.

Construction of the property, between Greenwich and Washington Streets, will begin soon, Ms. Thomas said. A one-story commercial building was torn down to make way for the project.