If Lee had commanded the Northern armies and won the war quickly, the Union would have been preserved with slavery still intact. How much longer would it have lasted under those conditions?
“manumission could be a lengthy process”
...And very expensive. Under VA law you had to put up a surety guaranteeing the freedman would not become a ward of the state - The surety was calculated to be enough to provide for all the former slaves life expenses. Thomas Jefferson spend much of his free cash with his projects building and rebuilding Monticello, he is said not to have freed his slaves due to this expense.