I saw him as a slave, (namely, Barira's husband).  

رَأَيْتُهُ عَبْدًا يَعْنِي زَوْجَ بَرِيرَةَ  

That was Mughith, the slave of Bani so-and-so, i.e., Barira's husband as if I am now looking at him following her (Barira) along the streets of Medina.  

ذَاكَ مُغِيثٌ عَبْدُ بَنِي فُلاَنٍ يَعْنِي زَوْجَ بَرِيرَةَ كَأَنِّي أَنْظُرُ إِلَيْهِ يَتْبَعُهَا فِي سِكَكِ الْمَدِينَةِ يَبْكِي عَلَيْهَا  

Barira's husband was a black slave called Mughith, the slave of Bani so-and-so-- as if I am seeing him now, walking behind her along the streets of Medina.  

يُقَالُ لَهُ مُغِيثٌ عَبْدًا لِبَنِي فُلاَنٍ كَأَنِّي أَنْظُرُ إِلَيْهِ يَطُوفُ وَرَاءَهَا فِي سِكَكِ الْمَدِينَةِ