Why do you think Amos buys the freedom for every one of his wives?
When Amos is captured and sold into slavery he is taken from Africa to America. During this event in his life he is taken from his family and is especially upset about leaving behind his little sister, Ath-mun. She is crippled and walks with a limp.
Amos spends his entire life looking for his sister. He wants to find her and if necessary to free her from slavery. He states, "by buying the freedom of the women he marries, he is in some way buying the freedom of his sister, if she needs it."
After securing his freedom he begins to look for a wife. Although he is now in his sixties he believes because of "fortune smiling on him" he has many more years to achieve his goals.
It is not good enough for Amos to simply marry the women he loves while they are slaves. It is more important for him to free them so that they too will know the joy of being free. When he purchases the freedom of his first wife, Lilly, for 20 pounds, her owner tells Amos that she is weak, sick and probably won't live a year. Amos tells him that at least she will die free. The second wife he takes, Lydia, costs him 50 pounds and he must work for years to buy her freedom. She also dies in a year after her freedom. His third wife, Violet, is younger that he is and also costs him 50 pounds to gain freedom for her and her daughter. All of these women are in someway connected to his desire to fulfill a promise he made to himself many years ago, not to forget his little sister.