Musannaf Ibn Abi Shayba 9:641

Ibn al-Mubarak narrated to us, from Hammad ibn Salamah, who said: If a slave woman is emancipated, and then he (her former master) has intercourse with her while she does not know that she has the choice (to remain or leave), then she has the choice. He said: And it reached me from al-Hasan that he used to say that.