Here's a version of that story I've never read (and I've read several). Note the date. The "Negro Soldier Bill" became law on March 13, 1865.

The Jackson Free Trader, of the 20th ult. [-March 20, 1865], says of Gen. FORREST: "We have just been informed that a few days since this truly noble man and distinguished officer called out twenty-six of his own negroes and said to them, all of you who are willing to become soldiers for the war, step forward and I will give to every one who does so, his free papers Twenty-five of them immediately advanced, only one refusing, who was a peace man, but he would drive a wagon for his master during the year." (my emphasis)

New York Times, April 8, 1865
http://www.nytimes.com/1865/04/08/news/north-carolina-affairs-wilmington-citizens-rejoicing-occupation-city-our-troops.html