Developer Gary Barnett of the Extell Development Company is seeking final approval for the city’s plan to help pay for a facelift to the sagging Diamond District in Midtown.

Tomorrow, the public is invited to testify about the plan to offer Extell up to $37.5 million in tax breaks over 10 years to build a 40-story commercial office building that the city and developer say will draw additional diamond businesses to the Diamond District on 47th Street between Fifth and Sixth avenues. The board of the city’s Industrial Development Agency will likely vote on the tax breaks next Tuesday.

Mr. Barnett’s partnership with City Hall follows a history of maverick actions during a meteoric rise in the city’s clubby real estate world to become one of the top developers.

In 2001, Mr. Barnett launched a series of lawsuits to derail the state’s plan to condemn property he owned to make way for a new 52-story headquarters for the New York Times on Eighth Avenue in Midtown. In one of the suits, Mr. Barnett charged that the taxpayer was subsidizing a sweetheart deal for newspaper and its partner, developer Forest City Ratner.

Last summer, Extell submitted the only competing bid for developing the Vanderbilt rail yards in Brooklyn, where Forest City Ratner is planning to build part of its $4.2 billion Atlantic Yards Project. The MTA awarded the site to Bruce Ratner, but not before the developer had to pay tens of millions more for the site than he originally planned.

Last year, partnering with the Carlyle Group, Extell bought about 77 acres of land on the Upper West Side owned by Donald Trump and several Hong Kong-based partners for $1.76 billion. Mr. Trump said he had been underpaid and sued Extell in an unsuccessful bid to recover more money. Just more than a mile uptown from that site, Mr. Barnett’s plans for two tall residential towers on an as-of-right site caused neighborhood outrage. The towers were built, but a city-led rezoning of the area followed. It included height limits.

In a sign that Mr. Barnett may be looking to work more closely with the city, Extell in March hired Mayor Bloomberg’s former deputy mayor of operations, Marc Shaw.

At tomorrow’s hearing, opponents of the city plan will testify that the city’s proposed subsidy would push Mr. Barnett’s rents below market rates, serving only to poach tenants from other buildings in the Diamond District.

A lobbyist representing some of the area’s building owners and tenants, Fran Reiter, a deputy mayor in the Giuliani administration, said Mr. Barnett’s application is “largely bogus,” as the types of tax incentives offered are normally tied to specific employers, not a developer that is promising to pass on the discounts to attract employers.

She said it was illogical for the city to incentivize commercial construction in Midtown, where the commercial real estate market is booming, with low vacancy rates and high rents.

“Why do you subsidize when the market is demanding more space?” Ms. Reiter asked.

City officials say the tax incentive offered to Mr. Barnett, a former diamond trader, is less than what he had initially sought and is tied to the number of jobs the building creates. They say adding discounted, concentrated, and modern space to the district would boost the stagnant diamond industry, which is on the verge of hemorrhaging jobs to other cities and countries.

An executive with Extell, Raizy Haas, said the incentives are required to keep the new tower, expected to cost more than $400 million, affordable to diamond industry tenants.

“It is my understanding that certain key companies in the industry are bursting out of their space, and their only other option is to move out of town,” Ms. Haas said.