The General Services Administration has received the green light from the National Capital Planning Commission to sell a 6.2-acre pentagonal tract of land on the District border with Prince George's County -- if the land can be kept as open space.

The property, located at 4900 LaSalle Rd. in Prince George's County, now is part of the Avondale Research Center, an arm of the Interior Department's Bureau of Mines.

GSA officials, however, said that the land would be put up for sale by competitive bids and sold to the highest bidder without an open-space restriction. One GSA official, who asked not to be named, said that the only way residents could maintain the open space is through local zoning decisions that would prevent the parcel from being used for a high-rise office building or condominium.

Diane Spencer, an area resident, said she and others plan to work with Rep. Steny Hoyer (D-Md.) and Sen. Charles McC. Mathias Jr. (R-Md.) to block the proposed sale. "Right now, we feel we're not in control of the situation," she said. "It looks like it might be an uphill fight."

Hoyer already has come out in favor of retention of the parcel by the federal government.

Until 1976, the property was owned by the Christian Brothers of the neighboring LaSalle College. Since then, the parcel that is up for sale served as the front lawn to the research center, bordering on Eastern Avenue, LaSalle Road and Carson Circle.