Hazrat Mujahid (may Allah have mercy on him) says: If a slave woman is divorced and she is freed during her waiting period (‘iddah), then if the divorce is one, she will observe the waiting period of a free woman. If there are two divorces, then there will be no return (ruju‘), and she will observe the waiting period of a free woman.
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2446