A Washington development firm has been given a 90-day option to buy the four-acre Shapiro tract in Adams-Morgan, a plot of land the city and community residents had hoped to turn into a park. The developer, Holladay Corp., would like to build 100 townhouses - priced from $65,000 to $100,000 - at the site.

Spokesman for Adams-Morgan, including City Councilman David A. Clarke, said they would try to block the sale.

Twice turned down by Congress in attempts to have purchase of the land authorized, the city was given permission this year to include money for tract in its budget.

After hearing about the sales contract, Mayor Walter E. Washington Tuesday told the City Council that he would like to offer $2 million for the property - a bid that would reportedly match the developer's. Maurice and J. B. Shapiro have owned the property, once the site of an apartment building, for nearly 20 years. The land has been vacant for several years.

Wallace Holladay acknowledged that the agreement had been signed, but said the sale was contingent on engineering, soil condition and economic feasibility reports.

Meanwhile, Councilman Clarke, who represents Ward One, where the tract is located, and Frank Smith, chairman of the Adams-Morgan Organization and head of the area's Advisory Neighborhood Commission, said that residents of the area would vigorously oppose development on the tract.

"The people up there have been looking forward . . . for some time" to using the land as a park and recreation center, Clarke said, adding that "if necessary" he would seek to use the city's power of imminent domain to forestall a sale to developers.

The vacant parcel, located between Calvert Street, Adams-Mill Road and Rock Creek Park is the last large piece of land in the Adams-Morgan community that could be used for recreation or housing for low and moderate income housing, Smith said.

In addition to the $500,000 in the 1977 capital improvements budget approved by Congress, the city planned to use $500,000 in federal grant funds and to transfer $1 million from other city building projects to buy the land.