No the Colonization project wasn't that successful. I believe that between 1860 and 1865 only about 3,000 slaves were purchased.

Nevertheless the ownership of slave within the District of Columbia was outlawed by proper legislative action in 1850. ten years before the War. That law even applied to Foriegn Diplomats who brought their slaves with them from other slave owning countries.

So it was possible to pass such laws throught the proper perscribed legislative process which included compensation of property taken. And I don't believe that anyone has claimed the slavery existed within the Washington DC environs proper during that period. Now Slavery existed all around Washington in the States of Maryland and Virginia. But in Washington itself is another matter.

That was a solution to the two part question that all the southerners asked. One, Am I going to be compensated for my investment in this work force? Two, What are you going to do with all the slaves, which would have been Non Citizens, after they were freed/purchased?

This is the same solution that had worked in England. The problem with this solution was two fold for the abolishionist. One, it was expensive even at $100 a head, even if you didn't colonize the free slaves back to Liberia. Two, It wasn't a fast enough process. A third problem not involving the Abolishionist was that Northern Industrialist would have loved the huge pool of cheap labor that freed slaves would have provided. The northern factory worker didn't. So there was no inclination even in the north to set up such a federal system nationwide. Which the federal government did not have the authority to do without the proper legislative consent of the individual slave holding states.