Craig You are correct. The 1850 date comes from the Missouri Compromise, which supposedly "ended" the slave trade in Washington DC. It was the 1862 act that abolished slavery by authorizing payment to slave owners for their slaves in Washibgton DC.

"Slavery in the District of Columbia ended on April 16, 1862, when President Lincoln signed a law that provided for compensation to slave owners. An Emancipation Claims Commission hired a Baltimore slave trader to assess the value of each freed slave, and awarded compensation for 2,989 slaves"

http://www.loc.gov/exhibits/treasures/trm009.html

Again no legislature, nor President had the Power under the Constitution to free any slave without compensation according to the 5th Amendment. However, such compensation was made by the United State government prior to 1862 in the way of appropreation by Congress to the Recolonization Society project not only in Maryland, but also to Virginia.

According to Harper's Weekly Lincoln supported a plan put forth by the recolonization society that sent those purchased to Liberia. And that is where most of those 2,989 slaves went. That is why President Clinton travel to Liberia during his Presidency and apoligized for American slavery.

According to the 1860 census the District of Columbia was home to 11,131 free blacks and 3,185 slaves. Seems like an awful small number of slaves compared to the Free black population and the population of Washington itself being a City between two slave holding states Maryland and Virginia.