Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Anas bin Malik (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) freed Hazrat Safiyyah and then married her. The narrator says that he was asked, "What did you give her as dowry?" He replied that he gave her herself as dowry, meaning her freedom was made her dowry.

Hazrat Ali (may Allah be pleased with him) says that if a man wishes, he can set his Umm Walad free and count her freedom as her dowry.

Hazrat Saeed bin Al-Musayyib says that if a man frees his slave-girl or Umm-e-Walad and makes that freedom her dowry, then I consider this act permissible for him. And (Imam) Abu Hanifa's opinion has been mentioned that: this marriage (of the freed slave-girl) will also be permissible with the dowry.