May one say Havdala early before Shabbos is over if he will be unable to say it after Shabbos?
If one will be unable to say Havdala after Shabbos, such as he must travel immediately after Shabbos for the purpose of a Mitzvah [or other purpose] he may say Havdala on Shabbos after Plag Hamincha prior to the conclusion of Shabbos. In such a case he does not say a blessing over fire [or Besamim] in Havdala, and remains forbidden to do Melacha until a row of three small stars become visible. He does not need to repeat Havdala even if the opportunity presents itself after nightfall. [However he is to say the blessing over Besamim and fire after Shabbos. Furthermore, he should repeat the blessing of Besamim after Shabbos even if he already said the blessing over Besamim on Shabbos during Havdala.]
Despite the above allowance, some opinions state it is proper for one to refrain from doing the above [i.e. saying Havdala and Maariv while still Shabbos] as it is a puzzling matter to the public.
If someone said or heard Havdala before Shabbos is over, prior to Bentching is he to say Ritzei in Bentching?
No. This applies even if he recites Birchas Hamazon prior to the leave of Shabbos.
 Admur brings that one has already began walking on Shabbos to the end of the Techum. Vetzaruch Iyun the necessity to say this.
 Kaf Hachaim 293/8
 See Nemukeiy Orach Chaim 293/1 which questions this ruling on the basis that how can one say “Hamavdil Bein Kodesh Lechol” if it is still Shabbos.
Perhaps however one can simply answer that Havdala is a praise to Hashem for separating Shabbos from Chol, and is not meant to actually separate.
 299/10, Vetzaruch Iyun why this was omitted in 293/2.
Other Opinions: PM”G A”A 299/1 sides that Besamim is said during Havdala which is done by day, as perhaps the extra soul leaves immediately after Havdala.
 Peri Megadim brought in Kaf Hachaim 297/23
 Bach in name of Rashal brought in Magen Avraham 293/4
 188/17; Ketzos Hashulchan 47/8
 Tzaruch Iyun, as in actuality it is still Shabbos for him regarding all matters. Hence if he bentches prior to the leave of Shabbos why shouldn’t he say Ritzei. If he bentches after Shabbos however it is clearly understood that he is to skip it. As the Havdala he made before Shabbos was over takes effect as soon as Shabbos concludes.