It's a huge show of kindness that Jenny offers to buy Isabel and Ruth. But it shows how powerless she is, even as a free white person, when the Locktons prevail anyway. Simply being white doesn’t make her powerful. In the same vein, Mr. Lockton insists that it’s essential for the Colonies to remain under British rule, as that’s the only way to maintain a social structure where someone like Jenny remains powerless. None of this matters much to Isabel, though, since she suspects that being sold to the Locktons is going to be dehumanizing and traumatizing.