The office portion of the double tower at 866 United Nations Plaza has hit the sales block and could fetch around $180 million.

Vornado Realty Trust has hired the “Queen of the Skyscrapers” Darcy Stacom, and her CBRE team to market the six office floors. The nearly 400,000 square feet can also be remeasured to reflect modern loss factors.

The building is 97.2 percent leased with numerous small- mission tenants. Among the largest is the Permanent Mission of Japan, which renewed its lease for 50,740 square feet in 2010 through Cassidy Turley.

Overall rents in 2011 were $52.41 per square foot, according to Vornado’s annual report.

“The tenants are sticky,” Stacom said. “They stay and renew and renew.”

Developed on land owned by Equitable Life as Alcoa Plaza by William Zeckendorf in 1968, it came to Vornado as part of the original Bernie Mendik portfolio.

A few years ago, sources said Vornado created an office-condominium plan but never forced the tenants to buy the spaces because of the tax consequences for a real estate investment trust.

Stacom said they are selling it as one piece but that a buyer could resell the individual office condominiums.

“This is a basis play, as the price per square foot is low and it has a good return,” she said. “The asset does not have debt, so you can take advantage of the strong financing market.”

Gil Robinov of Murray Hill Properties, who has represented tenants at the building, said it has always been attractive to foreign tenants but needs a sensitive buyer and good management — like Vornado’s — to work with the various foreign governments.

The rest of the 38-story twins are filled with river-view co-op apartments that have sold for up to $5 million.

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The Association Building at 120 Wall St. is getting a boost as Empire State Development has a new 20-year program that allows nonprofits to once again obtain property-tax exemptions at the Silverstein property.

The initial IDA Civic Facilities legislation program that began in 1994 ran out in 2008. To the dismay of nonprofits, unions insisted any new program include a provision that participating properties had to employ union workers.

Since then, without the tax break, 120 Wall has lost the Juvenile Diabetes Research Foundation, the International Planned Parenthood Federation, Guttmacher Institute and the Ms. Foundation for Women — all of which either bought condominiums or signed cheaper leases elsewhere.

Sources said that because Silverstein already employs union workers, this new building-specific program would not ruffle any mops.

Peter Olans of Trident Commercial represented the New York State Nurses Association, which is leaving 120 Wall next year. The group just paid $11,465,429 for 28,000 square feet on floors 3 and 4 of 131 W. 33rd St., through Brandon Madeiros at Time Equities.

“For the nurses, [the new program] wouldn’t have been a factor, as they only got half the benefits,” said Olans.

But Suzanne Sunshine of Sunshine & Associates, who represents nonprofits already priced out of the building, worries this may be too little too late.

“Silverstein will need to both significantly lower rent by at least the amount of the tax break and invest capital in the infrastructure and the build-outs of the new space,” said Sunshine.

The good news? Silverstein has hired Moed de Armas & Shannon Architects for a façade-and-lobby renovation to give 120 Wall “a fresher, more contemporary look.”

A spokeswoman for ESD said, “Effectively, this transaction ensured that the jobs and benefits these organizations provide — from AIDS research to elder care and community service — would not have to leave New York.”

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E-commerce site Fareportal, the parent of CheapOAir.com, grew so fast that every time brokers Corey Abdo and Kristy Park of Winoker Realty settled on a space, the company needed more folks and more footage.

Packed into 7,000 square feet now, the site went from seeking 15,000 square feet to 25,000 to more than 30,000 and finally into a 40,848 square-foot, seven-year sublease with an asking rent of $40 a foot. At that point they got cold feet and retreated to explore less pricey digs in Long Island City.

“The total package was a little daunting, as they were going from 7,000 square feet to 40,000 square feet,” said Abdo.

By the end of the summer, however, the execs were ready to ink the 5th-floor sublease from Alliance Bernstein at 135 W. 50th St. Come December, they will move from 213 W. 35th St. and other satellite spaces that house 186 people and consolidate at 135 W. 50th St., where they expect to house more than 300 workers by the end of next year.

“They are a call center and are densely packed,” said Abdo. The new space includes perimeter offices, an open bullpen area and 85 tons of extra cooling power that was previously installed. “It was a yearlong process, and the deal closed in two weeks,” Abdo said.

Facilitating that quick closing were Bob Alexander, Christopher Mansfield, Joe Mangiacotti and Scott Bogetti of CBRE, which repped Alliance Bernstein in the deal.

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