Her (Barira's) husband was a slave, so Allah's Messenger (ﷺ) gave her the option (either to retain her matrimonial relation with her husband or sever it off). She opted to break off (and secure freedom for her even from the matrimonial alliance). And if he were free he would not have given her the option. In the hadith narrated on the authority (of this chain of transmitters) these words are not found: Amma ba'du.