I agree with milanessa. And I would guess that the edition's not faked, but that a former slave was not the author. The whole "dictated" thing and that it appeared in a northern newspaper is what's tripping my alarm bells, and and it sounds far too educated for the *average* former slave. This isn't Frederick Douglas we're talking about here and he was an extreme outlier example of an educated freedman.
At the very most, I'd believe the general idea behind the letter may have come from a former slave whose old master sent a letter and the content, tone, and vocabular are all by an editor (my first guess would actually be someone not dissimilar from Douglass, a highly-educated and unusual literate former slave. Second guess is a former abolitionst who still was paying attention after the war, which wasn't most of them but was some.) At worst, I'm curious how much checking's been done to see if any of the named people in it actually existed and lived where they did.
Maybe I'm just overly skeptical because I deal with a lot of old first-person documents and 99% are...skewed, at best. (There's a reason eyewitness accounts of anything are lousy testimony and often disproven by things like archaeology. I'm doing Titanic work right now and it gets increasingly clear that witnesses fudge, didn't see what they thought they saw, or especially where the passenger survivors are concerned, didn't know what they were seeing or just made stuff up whole cloth. So my BS meter may be running on high right now.) But it's a little TOO perfect and appealing to be taken at face value.