“manumission could be a lengthy process”
...And very expensive. Under VA law you had to put up a surety guaranteeing the freedman would not become a ward of the state - The surety was calculated to be enough to provide for all the former slaves life expenses. Thomas Jefferson spend much of his free cash with his projects building and rebuilding Monticello, he is said not to have freed his slaves due to this expense.
In the case of two of Sally Hemmings daughters, they were give some money and told they could leave Monticello and that they would not be pursued. They both left and sorta disappeared into white society somewhere.