Bone and skin chicken; cut into bite-size pieces. In a large resealable plastic bag, combine 1/4 cup soy sauce, brown sugar, garlic and cumin if desired; add chicken. Seal bag; refrigerate for 4 hours or overnight. Drain chicken, discarding marinade.
In a large skillet, heat oil over medium-high. Cook chicken for 6-8 minutes or until juices run clear. Remove chicken with a slotted spoon; set aside. Saute celery in drippings for 2 minutes or until crisp-tender. Add the beans, water chestnuts and mushrooms; cook for 5 minutes or until heated through. Add chicken.
Combine the cornstarch, water and remaining soy sauce until smooth; stir into chicken mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with rice.