Is what the questioner said considered an act of slander (qadhf) or calumny (buhtan), and how can he absolve himself of his sin, and will his repentance be accepted?

As long as you did not accuse the woman explicitly of zina (fornication/adultery), this is not a qadhf (accusation of unlawful sexual intercourse) that necessitates a hadd (prescribed punishment). However, it is a sin, and its expiation is repentance to Allah Almighty. You are not required to inform the woman or seek her forgiveness. It is sufficient that you recant your words to your two companions, thereby denying what you attributed to the woman regarding illicit acts.