The former sentence means, "There is a restaurant in front of a post office"　 「前に」 would mean in this context "in front of" or "right across the street from" and the restaurant is on the other side of the street. The latter sentence means, "There is a restaurant before a post office" 「手前に」 would mean in this context "before" or "closer to the speaker" and the restaurant can be at the same side of the street as the post office.