The message saying 'Let this boy be discharged', and signed A Lincoln was written after a telegram from Colonel Thomas W Harris about his son, Perry.
It had been in a private collection and was valued at USD 15,000 by Nathan Raab of the Raab Collection, which offered the previously unknown document for sale in Philadelphia.
The letter is considered rare because there are few Lincoln documents relating to children, FoxNews.Com reported.
"It shows the type of person [Lincoln] was and how he was defined by clemency. When you see it's involving a father and his son, it strikes a personal chord and likely did for Lincoln. He was a father of four who lost all but one of his sons," Raab said.
Harris sent his telegram on February 6, 1865, to Illinois Senator Lyman Trumbull and General John M Palmer, pleading, "My son Perry Harris 14 years old insane crippled has been mustered in 55th Kentucky regiment. Please have Secretary of War order him discharged. Col Thos W Harris of Shelbyville, Ill."
