It requires that USA has researched State and Government and have 5000£ in the treasury. It also requires that Liberia is a puppet of the United States, which it is at the start of the game.
The decision releases Liberia as a puppet, and gives United States 150 influence, which effectively spheres Liberia if USA have not already done so. USA will lose 5000£ and Liberia will gain those 5000£.
It also gives Liberia research points, 15% modifier to research points as well as 5prestige.