The common phrase was "selling South". Pennsylvania IIRC set the maximum age for a slave born after a certain date at 27. That left the owner 27 years to sell his slave to an owner in a state that would allow that person to be enslaved for life. This had 2 benefits as far as Pennsylvania was concerned.
1) it fully compensated Pennsylvania slave owners for their property. They would get market value for all of their slaves. Thus they were not nearly as opposed as they would have been had their slave property simply been expropriated (emancipated).
2) It got rid of Black People. It got them out of state. Oh, I know that's a horrible, nasty, wicked and rude thing to say. Unfortunately, its also true. This was very much a concern of Northerners. They didn't just want to get rid of slavery. They wanted to get rid of Black People. That's why they passed the Black Codes - to make it extremely difficult for Free Blacks to live so as to drive them out too.
CW 5:534-35, President Lincoln, December 1, 1862, Annual Message to Congress
Heretofore colored people, to some extent, have fled north from bondage; and now, perhaps, from both bondage and destitution. But if gradual emancipation and deportation be adopted, they will have neither to flee from. Their old masters will give them wages at least until new laborers can be procured; and the freed men, in turn, will gladly give their labor for the wages, till new homes can be found for them, in congenial climes, and with people of their own blood and race. This proposition can be trusted on the mutual interests involved. And, in any event, cannot the north decide for itself, whether to receive them?