The story illustrates one of the play's main themes: exploitation. Wherever she goes and whatever she does in life, Rita Joe always finds herself being exploited, whether it's by white Canadian society or the people of her own community. Yet it's Rita Joe's exploitation at the hands of white folk that is the main focus of the play. We can see this in the tale of Sandy Collins' shocking attempt to buy Rita Joe from her father David when she was four years old. Sandy had recently lost his own daughter and figured that as David had another child on the way, he wouldn't miss Rita Joe. So he offered to take Rita Joe off David's hands for $1,000.
Thankfully, David has too much love for his daughter—not to mention basic decency—to accept such a squalid deal. But the whole shabby episode is important as it highlights just how much contempt white society has for the indigenous people. Sandy blithely assumed that family ties among the native tribes don't really mean all that much, and as such can be quantified in purely monetary terms. In assuming that David would gladly sell his own daughter, Sandy is projecting the values of modern society onto a pre-modern society where bonds between family members tend to be much stronger.
The primary significance of this story is that it conveys his idea of human worth, more particularly, the human worth of his daughter, Rita Joe. This contrasts to Rita's experience in the city because there, she learns she has no worth. She learns that while she would like to assimilate into the city life, it is virtually impossible for her because she is not already assimilated. This poses a paradoxical problem that undergirds the premise of the play.