The author of Zaad al-Mustanfi said it is permissible for the imaam to greet the congregation with salaam when he faces them. Shaykh Ibn Uthaymeen said, in his interesting comment, that this is because this practice was reported from the Prophet (peace and blessings of Allaah be upon him). Even though there is some weakness in this hadeeth (i.e. in its isnaad), this is what the ummah has done and it is well known that when the imaam comes and gets up on the minbar, he greets the people with salaam. (Reported by al-Shubi). Abu Bakr and Umar used to do this. Reported by Abd al-Razzaaq , Ibn Abi Shaybah, 2/114, Ibn Maajah and al-Tabaraani. It was also reported by al-Bayhaqi from Jaabir ibn Umar, narrating from the Prophet (peace and blessings of Allaah be upon him), then he said: a report concerning this was narrated from Ibn Abbaas from Umar ibn Abd al-Azeez. Then the shaykh said in his commentary: it is not permitted to speak when the imaam is delivering the khutbah, it is only permitted before and after the khutbah, even after the khateeb has arrived and after the adhaan, so long as the khateeb has not started his khutbah. (Ibn Uthaymeen, Al-Sharh al-Muttabi li Zaad al-Mustanfi, 5/78). [?? Makath ??] is well known, from which we know that it is obligatory to return the greeting of salaam, so in this case we understand that it is not forbidden to speak. And Allaah knows best.
Ibn Qudaamah said in al-Mughni: When the imaam faces the people, he greets them with salaam and they respond, then he sits down. Then he mentioned the hadeeth quoted above, and others. Then he said (may Allaah have mercy on him): When he greets them with salaam, the people should respond, because it is more of an obligation to return the greeting than to initiate it. Then he should sit down and rest until the muadhdhinoon (muezzins) finish. (al-Mughni 2/297). And Allaah knows best. May Allaah bless our Prophet Muhammad and his family.