Slavery exhibits at the Freedom House Museum in Alexandria, Va. The basement and yard served as a slave pen for as many as 10,000 people during the early 1800s. (Ricky Carioti/The Washington Post)

The National Trust for Historic Preservation awarded Alexandria a $50,000 planning grant to help preserve the Freedom House Museum, the former headquarters of one of the nation’s largest slave traders in the early 1800s.

The Northern Virginia Urban League, which owns the site at 1315 Duke St., and the city plan to expand interpretation, management and preservation of the museum, which was financially faltering when the city gave it a $63,000 loan earlier this year. The city sought a $125,000 grant from the National Trust as part of its effort to help preserve overlooked sites that showcase African American history. Sixteen projects around the country were awarded $1 million.

Gretchen Bulova, acting director of the Office of Historic Alexandria, said she was “thrilled” to be included on the list of winners but will have to scale back the original preservation planning unless the city can win some other grants. The museum, which is open Saturdays and Sundays from 1 to 5 p.m., has become a popular destination for group tours in the past six months, she said.