Her (Barira's) husband was a slave, so Messenger of Allah 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.

وَكَانَ زَوْجُهَا عَبْدًا فَخَيَّرَهَا رَسُولُ اللَّهِ ﷺ فَاخْتَارَتْ نَفْسَهَا وَلَوْ كَانَ حُرًّا لَمْ يُخَيِّرْهَا وَلَيْسَ فِي حَدِيثِهِمْ أَمَّا بَعْدُ

While relating the tradition about Barirah A’ishah said “her husband was a slave, so the Prophet ﷺ gave her choice. She chose herself. Had he been a free man, he would not given her choice.”

كَانَ زَوْجُهَا عَبْدًا فَخَيَّرَهَا رَسُولُ اللَّهِ ﷺ فَاخْتَارَتْ نَفْسَهَا وَلَوْ كَانَ حُرًّا لَمْ يُخَيِّرْهَا

“Barfah’s husband was a slave, so the Messenger of Allah let her chose, and she chose herself, and if he was a free man she would not have had a choice.”

كَانَ زَوْجُ بَرِيرَةَ عَبْدًا فَخَيَّرَهَا رَسُولُ اللَّهِ ﷺ فَاخْتَارَتْ نَفْسَهَا وَلَوْ كَانَ حُرًّا لَمْ يُخَيِّرْهَا

[AI] Her husband was a servant, so the Messenger of Allah ﷺ gave her the choice, and she chose herself. Even if he had been a free man, she wouldn't have chosen him.

كَانَ زَوْجُهَا عَبْدًا فَخَيَّرَهَا رَسُولُ اللهِ ﷺ فَاخْتَارَتْ نَفْسَهَا وَلَوْ كَانَ حُرًّا لَمْ يُخَيِّرْهَا