How does a person seek forgiveness for an act when they are unsure whether it is unlawful (haram) or permissible (halal), such as a conversation where they suspect it might have involved backbiting? And is there a precautionary repentance by saying: "O Lord, if I have erred, then forgive me"?

General repentance is required of a Muslim in every situation, and it is beneficial for the forgiveness of sins, whether known or unknown, mentioned or unmentioned. If a person does not know whether they have sinned or not, or has forgotten their sin, or fears that they may have sinned unknowingly, then they should make this general repentance. The best form of seeking forgiveness (Istighfar) is "Sayyid al-Istighfar" (the Master of Seeking Forgiveness): "O Allah, You are my Lord; there is no deity except You. You created me, and I am Your servant, and I am abiding by Your covenant and Your promise as much as I can. I seek refuge in You from the evil of what I have done. I acknowledge Your blessing upon me, and I acknowledge my sin, so forgive me, for indeed, none forgives sins except You." And by whatever formula a servant seeks forgiveness, the purpose will be achieved.