If you are going to argue this issue, you need to catch yourself up.
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
Though the word "slave" isn't mentioned in the US Constitution, that clause was put in there specifically to guarantee the return of slaves fleeing their masters.
It makes it virtually impossible to create a "free" state, and indeed, George Washington himself kept slaves in Pennsylvania after it had become a "free" state by simply rotating them in and out from his other plantations in other states.
Since the "free" states were constitutionally obligated to recognize the property rights of slave owners, how are you going to keep slave owners from coming into your state?
They had a right to do that under the US Constitution.
No, the problem was baked into the cake from the very beginning.
The Constitution acknowledges the existence of slavery in the United States in others articles of the Constitution also, But acknowledgement of its existence does not protected as a right. The Federal government end the importation of slaves, it could prevent slaves from being transported to the territories yet to be states. The lack of conferral of legal standing to the institution of slavery was a compromise made to get a Constitution that all of the colonies would buy into.