The master is frequently compelled to sell this class of his slaves, out of deference to the feelings of his white wife; and, cruel as the deed may strike any one to be, for a man to sell his own children to human flesh-mongers, it is often the dictate of humanity for him to do so;
Often, the masters would have children with enslaved black women, resulting in mixed-race children, also known as "mulatto". Because they were his children (master), and mixed-race, they were harshly beaten. The wife/white-mistress hated them, the master wasn't allowed to openly protect them. So, they were sold, which was a better outcome as Frederick claimed.