The point here is obvious to someone not brainwashed in deliberate propaganda. The point is that the Union had no concern for the lives or rights of slaves until it became good propaganda, good war tactics and good politics to have a concern for their lives and rights.
The Union tolerated their condition for four score and seven years, until a piece of it wanted to leave, then suddenly what was completely tolerable for them became an ex post facto casus belli. It gave them an after the fact justification for chasing down and beating that runaway slave.
This is what is known as "selective morality." Slavery only became objectionable to the Union when it was needed as a tool to justify the abuse they put the country through. It's apologists have been hyping it ever since.
But the point, and I'll have to trust you when you say you had one, was irrelevant to the question at hand. Which was the 3/5ths clause.
As usual you have it backwards. It was the south's "casus belli" not the north's.