Which was a reason that northern leaders disagreed over the south
rejoining the union?

This may not be entirely correct, but the North didn't want to the South to join the Union because they highly disagreed with the idea of slavery, which is what the South believed was a good thing since it benefited them. Although, the North also wanted them to join the Union when slavery was no longer a thing because they were cut in half as a country and were more vulnerable to Britain and any other country who wanted control over the United States.

Explanation:

missouri compromise of 1820 which prohibited slavery