Sayyidah Aishah (may Allah be pleased with her) said that Barirah's husband was free, and when Barirah was freed she was given the choice. She said: I do not like to stay with him, even if I were to have such and such.
Al-Thawri inserted it like this into the hadith from Aishah (may Allah be pleased with her); the statement "her husband was free" is the words of al-Aswad, not the words of Aishah (may Allah be pleased with her), as the following shows.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14274