You would have thought it was in the National Archives already: "A letter handwritten by Gen. Ulysses S. Grant to Gen. Robert E. Lee on April 10, 1865, discussing terms of surrender of the Confederate Army and originally penned at the Appomattox Courthouse in Virginia, will be sold the weekend of August 4-5 by Gallery 63."
On the other hand, it could be a fake. Even if it "was authenticated by the late Civil War historian Shelby Foote."