kiddushin 21b - retziah for kohen

the braisa says that a kohen cant begome a nirtza because it will make him a baal mum. the gemara asks, so what if he turns into a ba'al mum. The gemara answers that he has to be ableto return back to his original status after he goes free which for a kohen includes able to do avoda. it seems that the gemaras original question is just 'so what if he becomesva baal mum' - is there any issurto make a kohen a bal mum. But rashi adds to the question that there is a mitzvah to do retzia. why does rashi need to adt this? rashi implies that for a kohen o make a mum without mitzva would surely be an issur, presumabely bec. he would be passul l'avoda (or maybe because of chovel b'atzmo). But since here is a mitzva, there isn't any issur mitzad hilchos mumin that would prohibit making a mum - so the gemara asks that he should be allowed to make a mum for mitzva. To this the gemara respondsthat there is a requirement in hilchos avodim that if he can't return to his original shteller then he can't be a nirtza.