The land is owned by Ralph and Millie Shull, who will continue to live in a ranch-style house adjoining the site.
The Shulls have been trying for years to preserve the land, which during the Battle of Antietam represented the right flank of the Union troop line led by Gen. Joseph Hooker.
The National Park Service hasn't had the federal funding to buy the land, said Antietam Superintendent John Howard.
But the Shulls are anxious to sell the land, or at least the development rights, to see them through their retirement. Ralph Shull is retired from the U.S. Department of Energy and Millie Shull teaches dance at Hagerstown Junior College.
At HJC, Millie Shull met professor Tom Clemens, who is president of the Save Historic Antietam Foundation.
The foundation's board of directors was unwilling to commit, since it is still paying off the preservation of the Grove Farm. But Clemens helped work out a deal with the association, a national organization.
The association has three months to close on the deal - a $110,000 purchase price to be spread over 10 years. The association is looking for financial partners, Frye said.
Eventually, the association will donate the land to the park service.
Clemens said the land is very important to Antietam's preservation.
"It's sort of the hole in the donut in that particular part of the field," because it's surrounded by land already protected, he said.
The association has been eager to preserve land in Washington County ever since it moved its headquarters to Hagerstown last May, Frye said.
The timing is perfect because the association's 10th annual convention opens today at the Four Points Hotel, formerly the Sheraton, on Dual Highway.
Nearly 600 association members who have signed up for the convention will get to see their dollars at work, he said.
This acquisition also will be the association's first in Maryland, although it contributed $15,000 to the Grove Farm in 1992. The state has spent far more than any other on Civil War preservation - $14 million, he said.
The deal also is personally satisfying for Frye, who began leading tours at Antietam when he was 12 and has led thousands of visitors past this point.
"It is so important when coming into a battlefield area that you have a sensory experience that transports you back into time," Frye said.