A story along these lines could be insanely gripping if done properly. I'm no screenwriter, but as I do have a history degree, read tons of stuff on the civil war, and enjoy narrative fiction, I had the thought many years ago that you could make an awesome movie out of this.
Take a black runaway slave, in the vein of Harriet tubman, coming back to the south to free his family, pursued by a bounty hunter, set against the Union advance into the south.
As he comes back north with the fam, the bounty hunter catches him at a union camp. Rain falls, union soldiers shrug their shoulders and refuse to assist the bounty hunter ( set it Round 1863-ish), and it's one on one with blunt weapons fighting for freedom.
That'd be a solid 2 hr movie. Dunno where Quentin took it, but I hope it had some of that.