2. Question: Why on Yom Kippur are you allowed to talk about what you are going to eat after the fast but one cannot talk about business on Shabbos?
A. Both things are forbidden on that day. One cannot eat on Yom Kippur, and one cannot do business on Shabbos (or Yom Kippur).
B. The Mishna Berura (307:1:1) says that the prohibition from the days of the Prophets against speaking about mundane things on Shabbos is that one is not allowed to speak on Shabbos about things he is forbidden to do on Shabbos since one has to feel as if all his work was completed by Shabbos, therefore the Rabbis enacted that one also cannot even talk about work which is forbidden to be done on Shabbos.
Answer: This prohibition just doesn’t apply to eating on Yom Kippur because one has no obligation to view himself as if he is satiated and has no need to eat on Yom Kippur, neither does fasting have anything to do with work, therefore there was no Rabbinic enactment on Yom Kippur prohibiting talking about eating after Yom Kippur (See Dirshu Mishna Berura 611:2:5:6, in back page 49).