Must we offer the same position to a temp worker returning from FMLA leave?

Q. We lease some of our workers from an agency. The agency handles all personnel needs of its employees, but our supervisors provide day-to-day direction to the workers. One of our agency employees, Jane, informed the agency that she was taking FMLA leave for her pregnancy. While Jane was out on leave, the agency supplied us with a replacement, Mary. Jane’s FMLA leave is over, and the agency wants to send her back. We would prefer to keep Mary. Does the FMLA oblige us to let Mary go and bring Jane back? ...