MB 43: And need not wait - Because he has already been appointed for this.
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53:16. A person who is not the regular Chazan must (44) decline a little before going before the Holy Ark. He shouldn't decline too much, rather, the first time he is asked he should decline, when the request is repeated a second time he should prepare himself as if he wishes to get up (i.e., to lead the service), and when the request is repeated a third time he should get up to lead the service. If the person who asks him to lead the service is a distinguished person [i.e. the Rabbi, one of the Rabbis in the Yeshiva, or a recognized scholar], he should not decline the request (45) at all.
MB 44: Decline - If a regular Chazan no longer wished to be Chazan and another was appointed in his place, the former Chazan does not need permission from the congregation to lead the service, since he can continue to be Chazan if he wishes. However, this applies only to the congregation; he does require the permission of the new Chazan. Thus is it written in the Responsa of Yad Eliyahu (Rabbi Eliyahu ben Shmuel, early 18th cent.) But according to what we have explained in the Biyur Halacha (by the author of the Mishnah Brurah), such a Chazan requires the prior permission of the congregation as well.
MB 45: At all - Since one should not decline a distinguished person. Tosafos (Talmudic commentary by rabbis of the first few generations after Rashi, who lived until the beginning of the 12th cent.) writes at the end of the seventh chapter of Tractate Pesachim (86b) that in a matter of crassness and pretentiousness, one should decline even the request of a distinguished person [to participate].
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Lawton Cooper LX5@nihhfed1.gov sha-58.01
THE LAWS OF READING THE SHEMA (K'RIAS SHEMA)
[We are now starting, with Hashem's help, the study of the laws relating to the Shema and the Blessings before and after it.