Tobacco is cultivated, but after the Revolution, British trade goes to the Caribbean. In OTL, many slaves were actually freed because the tobacco business had matured and did not need the extra labor. Even George Washington thought slavery would be a dying practice in upcoming years. I am sure this logic led John Adams and many northerners to accept slavery for the sake of uniting the former colonies.
The POD would be to kindle a sentiment in the 1780's to phase out slavery by making all people born in the U.S. free citizens.