What is the required waiting period ('iddah) for a female slave who gains freedom while married to another slave, if she chooses to separate from the marriage?

Primary text

If a female slave is freed while under a marriage contract with a slave, and she chooses to separate, she must observe the waiting period ('iddah) of a free woman. This is because she becomes separated from her husband as a free woman. Evidence for this ruling includes the narration that the Prophet, peace and blessings be upon him, commanded Burayrah to observe the waiting period of a free woman.