This Ayah states the prescribed punishment for making false accusations against chaste women, i.e., those who are free, adult and chaste. If the person who is falsely accused is a man, the same punishment of flogging also applies. If the accuser produces evidence that what he is saying is true, then the punishment does not apply. Allah said:
(and produce not four witnesses, flog them with eighty stripes, and reject their testimony forever. They indeed are the rebellious.) If the accuser cannot prove that what he is saying is true, then three rulings apply to him: (firstly) that he should be flogged with eighty stripes, (secondly) that his testimony should be rejected forever, and (thirdly) that he should be labelled as a rebellious who is not of good character, whether in the sight of Allah or of mankind.
Then Allah says:
(Except those who repent thereafter and do righteous deeds; (for such) verily, Allah is Oft-Forgiving, Most Merciful.) This exception refers to the second and third rulings mentioned above. The flogging has been carried out regardless of whether he repents or persists, and after that there is no further punishment, as is agreed among the scholars. If he repents, then his testimony may be accepted, and he is no longer to be regarded as a rebellious. This was the view of Sa`id bin Al-Musayyib -- the leader of the Tabi`in -- and also a group among the Salaf. Ash-Sha`bi and Ad-Dahhak said, "His testimony cannot be accepted even if he does repent, unless he himself admits that he said something false, in which case his testimony may be accepted." And Allah knows best.