Qasim bin Muhammad narrates from Sayyidah Aishah (may Allah be pleased with her) that there were three sunnahs, or ways, concerning Sayyidah Barirah (may Allah be pleased with her); one of these sunnahs was that she was given the choice (about her husband) after she was set free.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13959