How was Sally Thomas able to free her sons in In Search of the Promised Land?

In In Search of A Promised Land, Sally Thomas, an enslaved black woman who is able to operate as a quasi-free woman, is able to free two of her three sons through buying their freedom. While Sally is never able to become free of the horrors of slavery, she is able to work semi-independently of the evil man who owned her by operating a laundry business and, through this, is able to earn enough income to buy two of her sons' freedom. The youngest and oldest sons, both of whom she is able to free, go on to become successful barbers. Her middle son escapes from slavery in Canada and eventually returns to Mississippi, where he also works as a barber.