The single-page letter, written by Lincoln to New York Herald publisher James Gordon Bennett on Sept. 28, 1861, has been given a value of $55,000 by the dealer, The Raab Collection.
Lincoln penned the letter, marked "Personal and Confidential," after Secretary of the Navy Gideon Welles refused to issue press credentials to Herald reporter Henry Wikoff, who had written unflattering articles about the war effort.
Other reporters had received the credentials, allowing them access to a Union warship on the Potomac River. Lincoln, whose relationship with the press was often contentious, in the letter compliments the paper's coverage and acknowledges that the Herald, which covered the Civil War extensively, could make or break the war effort.
"I write this to assure you that the administration will not discriminate against the Herald, especially while it sustains us so generously, and the cause of the country so ably as it has been doing," Lincoln wrote.
The letter was recently acquired from its unidentified longtime owner by The Raab Collection, based in suburban Philadelphia. It is the first time the letter has been offered for sale to the general public, the dealer said Wednesday.