Musannaf Ibn Abi Shayba 7:465

Yazid ibn Harun narrated to us, from Juwaybir, from al-Dahhak, regarding *zihar* (the pronouncement of one's wife as unlawful like one's mother) involving a slave woman: If he does not find what he can emancipate, and he is not able to fast, and he wishes to marry her, he makes her emancipation her dowry. Thus, her emancipation is the expiation for the *zihar*, and she becomes his wife.