Complex sentence: If he wins, Iachimo will get Innogen's ring from Posthumus's finger; if Posthumus wins, not only must Iachimo pay him but consent to a sword duel so that Posthumus may avenge his affronted honor.
Split into simpler sentences: If he wins, Iachimo will get Innogen's ring from Posthumus's finger. If Posthumus wins, not only must Iachimo pay him but also consent to a sword duel so that Posthumus may avenge his affronted honor.