Part of Lincoln's instructions were realistic and part were not. Overall, Lincoln told the freed slaves two things. Here is the quote:
And I hereby enjoin upon the people so declared to be free to abstain from all violence, unless in necessary self-defence; and I recommend to them that, in all cases when allowed, they labor faithfully for reasonable wages.
I would argue that the first part was realistic while the second was not.
It was realistic to tell the freed slaves not to be violent. The slaves were probably not in position to do much in the way of taking violent revenge against their former masters. In addition, it would be completely possible (if perhaps difficult) to simply leave without feeling the need to take revenge.
However, it would have been practically impossible for the ex-slaves to work for wages. The idea that the Southerners, still in the middle of the war, would accept the idea that the slaves were free and start paying them wages is ridiculous.