A developer is buying two office buildings in the financial district for more than $100 million. Until recently, the buildings were part of a site where a new trading complex for the New York Stock Exchange and a 50-story skyscraper had been planned.

Instead of a tall tower, the developer, A. I. & Boymelgreen, plans to convert the buildings at 23 Wall Street and 15 Broad Street into as many as 500 apartments, with retail stores and some office space.

The purchase is the latest example of the residential building boom downtown, where developers are planning the creation of more than 5,000 apartments over the next three years. Mayor Michael R. Bloomberg has promoted the trend as a way to revive Lower Manhattan, which has suffered corporate relocations and layoffs since Sept. 11.

A. I. & Boymelgreen signed a contract on Monday to buy the properties from J. P. Morgan Chase for more than $100 million, after winning an auction with a bid considerably higher than those of other developers, real estate executives say.

Asi Cymbal, vice president and general manager of Boymelgreen, confirmed that his company had signed the contract but declined to discuss the sale price, as did J. P. Morgan Chase.

‘‘We’re excited,’’ Mr. Cymbal said. ‘‘We look forward to contributing in a material way to the revitalization of downtown Manhattan. We along with many others fled downtown after Sept. 11. We look forward to coming back.’‘

He said the development is a joint venture between Boymelgreen, a Brooklyn company, and Africa-Israel Investments, a publicly traded Israeli company. Mr. Cymbal said they would preserve the landmark building at 23 Wall Street, the original headquarters of Morgan Bank, and renovate the other structure.

‘‘There’s going to be a lot of residential units produced downtown, which I think is a good thing for the city,’’ said H. Henry Elghanayan, a principal at Rockrose Development Corporation, which is building 650 apartments at 2 Gold Street. ‘‘The administration is in favor of it.’‘

In recent years, the city and state had hoped to demolish 15 Broad Street and several other buildings on the block, including one owned by Rockrose, to make way for a $1 billion trading complex for the stock exchange and an office tower. The city eventually spent $109 million putting the project together, but it fell apart last year when the exchange pulled out. But J. P. Morgan Chase had already emptied its buildings. The bank quietly put the buildings on the market in January.

‘‘This has been a particularly uncertain site since the demise of the stock exchange,’’ said Carl Weisbrod, president of the Alliance for Downtown New York. ‘‘The interest in the property and the ultimate disposition are a welcome sign for downtown and this historic intersection.’‘

The new owners can boast that the apartments will sit across the street from the stock exchange. But that may also be a problem; residents and businesses in the area have complained about the blockades and security details in front of the stock exchange giving the neighborhood a war-torn ambience.