This is an excellent question. What is interesting about how Dana returns is the way that so little comment is made about it by the narrator. It is as if we are left to work out the meaning of Dana's amputation for ourselves and try and establish what Octavia Butler was trying to communicate through it. Interestingly, though, in an interview with the author, Octavia Butler elaborates on the silence left in the novel concerning Dana's amputation as she returns back to the present for the last time, leaving her experiences of slavery in the past. Note what she said to explain the amputation:
I couldn't really let her come all the way back. I couldn't let her return to what she was, I couldn't let her come back whole and that, I think, really symbolises her not coming back whole. Antebellum slavery didn't leave people quite whole.
Thus it is that Dana's amputation therefore symbolises the way in which slavery broke people, and left them fragmented as individuals, in the same way that Dana finds herself literally fragmented, as she loses her arm. Butler seems to suggest that Dana is not just a spectator. As a participant and somebody who has suffered the horrors of slavery herself, she is not allowed to return to the safe present and left to imagine that her experiences were just a dream. Her lack of an arm will act as a constant reminder of what she and her ancestors suffered, symbolising the profound inhumanity of slavery.