On her journey of love, old Phoenix encounters several impediments to her progress, one of which is her fall and subsequent encounter with the young hunter. When he first sees her, he laughs and asks, "Well, Granny!....What are you doing there?" Then, he lifts her out of the ditch in which she has fallen and gives her a swing before setting her down. "Anything broken, Granny?" After asking her where she is going, the white man laughs, "I know you old colored people! Wouldn't miss going to town to see Santa Claus!" Clearly, this man is patronizing the old woman and being derogatory about her race.
So, while Phoenix scolds herself, "I come to stealing," when she surreptitiously picks up the man's nickel as he sics his dog on the big stray that comes at her, there is a certain sense of receiving retribution for the ridicule of the white man. Then, when he lies, "I'd give you a dime if I had any money with me," Phoenix feels even more justified in her theft.
Most readers, then, would understand the poor old woman's taking of this nickel, a small coin whose loss will not affect the hunter, but means something to her. While she herself is aware of her dishonesty, she probably feels that the white man owes her for his fun at her expense.