The Brits offered Emancipation to Colonial slaves in return for their help in defeating the colonists.  It didn’t work then and it didn’t work in the War for Southern Independence…

“Lincoln had it in his power to end his war upon the American people in the South and return the two country’s to a peaceful condition. Facing an unpopular war and dwindling enlistments, he resorted to the same incitement of slave insurrection as Lord Dunmore, Royal Governor of Virginia in 1775, did in order to defeat the American colonists bid for political independence. 

As George III used Hessians against the colonists, Lincoln’s emissaries scoured Ireland and Europe for soldiers kill Americans for bounties and eleven dollars a month.”

Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission