The quote in question can be found in chapter 5. Master Lockton says his freedom and liberty quote as a rebuttal to Bellingham's own usage of the two terms. Bellingham says that Lockton has come home to fight against the people that strive for freedom and liberty, and Lockton responds by stating that freedom and liberty have many meanings. As much as it pains me to admit it, Lockton is correct. That's what helps to make him such a great antagonist. He is smart.
One thing to consider for your essay is to explain to your reader how freedom and liberty mean different things. In present day society, the two terms are often used interchangeably; however, they do have different meanings, and Lockton and Bellingham would absolutely have known the difference between the two terms. In a broad generalization, freedom is the ability to do what you please when you want to do it. Liberty is freedom to do things that the laws of the time say you can do.
Knowing that difference could propel your essay into different directions. On one hand, Isabel doesn't have the freedoms and liberties that Lockton has. She can't because she's not treated as a human deserving human freedoms and liberties. Isabel is an object. She is a piece of property to be bought and sold; therefore, freedoms and liberties do not apply to her the way that they apply to Lockton. Isabel would obviously disagree, but the laws of the land would uphold that kind of reasoning.
On the other hand, Isabel does indeed have some of the same freedoms and liberties as Lockton and other members of society have. She is at liberty to go to the Tea Water Pump. Lockton has that same liberty. Lockton and Isabel are both free to leave New York (and Isabel does this near the novel's conclusion), but Isabel is not at liberty to do so. She would be breaking the law by becoming a runaway slave, so while Isabel does have freedom and liberty, the liberties that she is granted are far fewer than those given to Lockton and other non-slaves.