On Hoshana Rabba we wish people a Gmar Tov. The Minhag Yisroel Tovah brings from the Eretz Tzvi that this means that although our fate, whatever it is, is already sealed, it should turn out for the best. Koheles says sometimes great wealth is bestowed upon his person for the bad. We therefore wish each other that whatever they were granted should be for the good.
The Darkei Chaim V'Shalom says we should not wish people Gmar Tov because the word Gmar means "final end" and telling someone to have a "good end" is quite ominous sounding. He suggests saying Gmar VChasima Tova.
Her also says that when someone give you a bracha you should not respond "V'Chein L'Mar", as in same to you. Mar also means bitter and we do not want to wish the person giving us a bracha that he should have a bitter year. For a response to a bracha, he recommends saying "Gam Atem".
The Chasam Sofer however was very makpid that when he gave bracha to his talmidim they should answer the words V'chein L'Mar. The source for this is the gemara (Megila 27b), where Rav complained to someone whom he gave a bracha to that he did not say V'Chein L'Mar.
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