There were slaves in Northern States already.
Yes there were. What’s your point? The fact is that each of the northern states had a plan or timeline for the eventual abolishment of the practice. It is noteworthy to mention that zero of the southern states had similar plans - or even an incentive to discuss such plans.
By 1860, a handful, here or there.
Source for statistics on numbers of slaves.
Since the American Revolution, beginning with Vermont in 1777, every Northern state gradually abolished slavery, such that by the 1840 census, only about half of Northern states still reported a few dozen slaves each.
But by the 1860 census, only New Jersey still had 18 slaves, Nebraska 15 and Kansas 2.
In 1860, no other Northern states reported slaves.
But the huge issue for Northerners was the Supreme Court's Dred-Scott decision, which effectively made slavery lawful in every state, North or South, by ruling that Africans could not be citizens, and slave-holders could take their slaves wherever they wished.
That, along with the 1850 Fugitive Slave Act, helped spark the Northern revolt against Federal enforcement of slavery, which in 1860 elected its first President, Abraham Lincoln.