The April 18, 1864 letter from Mary Todd Lincoln requests the services of a White House baby sitter to look after the Lincolns' 11-year-old son, Tad.
The Raab Collection in Philadelphia has set the price at $15,000 for the one-page handwritten letter, which belonged to an anonymous private collector.
Raab Collection vice president Nathan Raab says the baby sitter, presidential messenger and valet Charles Forbes, would be blamed by the first lady for her husband's assassination a year later.
Forbes accompanied the Lincolns to Ford's Theater and allowed assassin John Wilkes Booth to enter the door to their private box.
