NEW YORK — A glass-enclosed 7,400-square foot penthouse in lower Manhattan sold for $30 million, a 13 percent discount that shows some of the borough's luxury developers are willing to negotiate price.

The 13-room TriBeCa neighborhood duplex was originally listed for $34.5 million, said broker Sean Murphy Turner, executive vice president for New York-based Stribling Marketing Associates. The penthouse had been under contract since August and closed earlier this month. She would not name the buyer.

"When you build something that is extraordinary and exciting, something like that may never come along again," Turner said.

The median price of a luxury apartment in Manhattan, defined as the top 10 percent of sales, fell 3.9 percent to $4.13 million in the fourth quarter compared with the same period in 2007, according to data from appraiser Miller Samuel Inc. Inventory climbed 26 percent to 1,730 properties and sales fell 9.5 percent. Rising inventory and falling sales preceded a drop in home prices nationwide.

Luxury units in Manhattan stayed on the market an average of 169 days in the fourth quarter of 2008, 52 days longer than it took to sell them in the last three months of 2007.

The TriBeCa apartment that just sold sits atop a historic building at 145 Hudson St. It has four bedrooms, 4 1/2 bathrooms, and three fireplaces encased in a "floating wall," according to a description provided by Stribling.

The unit is surrounded by a 4,400-square-foot wrap-around terrace with views of Manhattan and the Hudson River. Amenities include a spa and outdoor kitchen, according to floor plans.

Designers scaled back the duplex after the New York City Landmarks Preservation Commission ruled in 2005 that it had been built too high, according to agency documents and Joseph Pell Lombardi, the original architect.

"They did have to scale back, but they could make it all glass, which in a way was positive thing," he said.