The District of Columbia's urban renewal agency yesterday agreed to sell an acre of land in Southwest Washington at the bargain price of $9 a square foot to the National Business League, the nation's oldest and largest black business organization.

The league, headed by Theodore R. Hagans Jr., the developer of Fort Lincoln New Town in far Northeast Washington, plans to build a new $8 million to $10 million national headquarters on the site at Ninth Street and Maine Avenue SW. The NBL is now located in a small building in upper Northwest.

"We are obviously delighted with the potential this project holds," said Sylvester Bass, the NBL executive vice president. "The establishment of a national headquarters building, which has been an official mandate since 1949," furthers the self-help and self-sufficiency philosophy of NBL founder Booker T. Washington, Bass said.

The five-member Redevelopment Land Agency board agreed to sell the 1.25-acre site to the NBL for $488,000 or $9 a square foot.

Land sold privately in Southwest is currently bringing $25 to $30 a square foot, according to a large city real estate and leasing firm.

Jackie Moore, an RLA spokeswoman, said the land was sold for a low price because it was an urban renewal parcel designated for use by a nonprofit organization.

The 79-year-old black business group, which has fought for owners of small black businesses throughout its history, was one of three applicants for the tract. The site is one of the last large parcels in the Southwest urban renewal area, the city's oldest urban renewal project, which began in the early 1950s.

The RLA board last October selected NBL over the Girl Scouts of America and the Second Baptist Church as the tentative developer, and yesterday gave its final approval to the project.

The new three-story, glass-and-precast concrete headquarters, which will be completed in early fall 1982, will allow the NBL to move out of its cramped small offices at 4324 Georgia Ave. NW and consolidate with its research branch, the Booker T. Washington Foundation, which is now located at 2000 K St NW.

Hagans, who was elected NBL president two years ago after the death of Berkeley G. Burrell, the league's president for 17 years, is a close friend of D.C. Mayor Marion Barry and city housing director Robert L. Moore. Moore, a member of the RLA board, did not vote yesterday on awarding the parcel to the league.

Hagans has been involved in several business deals with the city during the last two years. In that period, the city spent 40 percent of its housing acquisition funds to purchase properties from Hagans. In addition, the city housing department took the unusual step of agreeing to subsidize the purchase of nearly two dozen $70,000 town houses by moderate-income families at Hagans' Fort Lincoln New Town. Hagans had been unable to sell the homes on the private market.