From the Will of Isaac Royall Jr., 1778:

To my daughter Mary McIntosh Erving “… the gift of my Negro girl Barsheba and her sister Nanny to her and her heirs forever and I do also give unto my said daughter my Negro woman Belinda in case she does not choose her freedom if she does choose her freedom to have it, provided she get security that she shall not e a charge to the town of Medford.”