Slavery must be phased out. After this particular slave has learned a trade to support himself, then he will be freed. The abolition of slavery was the core issue around which libertarians of the early 19th century rallied. They opposed phasing out slavery just as they would have opposed phasing out rape. A core issue around which modern libertarians must rally is the abolition of state coercion, as fast as is humanly possible.
Libertarianism is the political philosophy based on the principle of nonaggression.
Every human being is a self-owner with inalienable rights. And gradualism is inconsistent with the moral foundation of libertarianism. Before proceeding, it is useful to distinguish gradualism as a policy from gradualism as a fact of reality. This latter form of gradualism says that, try as you may, it takes time to implement ideas. This is the nature of the temporal reality in which we live.