PBA: B’shulei Hamichtav:
We know that a Kinyan Kaspo can eat T’rumah. However, Kinyan Kaspo is clearly an outdated concept as no one can morally claim an ability to own slaves nowadays. However, we cannot take away the right from those who were once able to eat T’rumah. Therefore, anyone who was ever a slave to a Kohen can eat T’rumah even after he or she has been freed. And since a person must feed his kids, they pass the ability to eat T’rumah to their descendants. Also, since slavery is a deplorable act which the Torah could never actually endorse, we must state that anyone who was ever a slave is retroactively freed. Thus, if anyone can prove that an ancestor of theirs (by patrilineal descent for a man and matrilineal descent for a woman) was once owned by a Kohen, he or she may eat T’rumah as well.