Snopes has confirmed the letter was printed in the 1860s in a couple of newspapers, and they have confirmed the existence of the stated parties. The people actually lived, and the letter was printed during that time period.
How much the letter was “massaged” by the transcriber or altered for publication is not known. There may have been anti-slavery/whatever motivations to improve the letter, such motivations being demonstrated by the publication of the letter in newspapers.