Why doesn't Tom leave when he has the chance or the 500 hundred dollars?
There are two answers to this really excellent question. To draw from within the text, we can say that Tom's is noble, good, and a Christlike figure who sacrifices himself for a white man's salvation. In general this is illustrated by the trend that he never puts up any resistance to his fate. He is accepting and finds it is better to be wronged than to do wrong.
However, viewed from a historical perspective this really compromises the text and can be regarded as a flawed authorial vision. Uncle Tom's Cabin is a protest novel, meaning it has a social agenda of not only illuminating the evils of slavery, but also changing them by appealing to the readers emotions ( a Sentimental Novel). Abolitionists at the time ,such as Beecher Stowe herself, repeatedly stressed that slavery was inhumane because it breaks up the family. Hence, it should logically follow that Tom's desire to return to his family is the same as any white person's. That is not the case. His desire for freedom is less than a white man's. Essentially then Beecher Stowe accords Tom just a little less humanity than she does her white characters, and that is quite telling. No doubt, the novel is important, but it is still influenced by the spirit of the time.