When Linda finally reaches the north she is only faced with a disappointment. The utopia she had dreamt of while lonely living in the attic quickly crashed as she experienced racism in the north similar to the racism she felt in the south as a slave. Physically she was living a "free" life but the way whites treated her was no different. She was not forced to follow rules regarding her skin color, the complete opposite of what she had imagined for herself in the north. In some aspects the south was even better, yes she didn't have to live with the fear of a beating as a punishment, but the people in the South were honest about their immoral behavior, where as in the north they denied it.
She gains her freedom by being bought, the same exact way her freedom was held from her in the South. She was fleeing the south to find a life better for herself where she could live freely around people who didn't hold her as a second class citizen but the north only offered the same flaws, just in a less upfront manner.