Musannaf Ibn Abi Shayba 9:1342

Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, that he used to say: If a slave woman is emancipated while she is under [the marriage of] a slave, and she is given the choice, and she chooses herself before he has consummated the marriage with her, then there is no dowry for her, and it is an irrevocable divorce.