How to return salaams to one who offers his hand in greeting during the khutbah
Question

If the imaam is preaching and someone greets you and offers his hand, what is the ruling?
Praise be to Allah.Gesture to him if it is the time of the khutbah, and put your hand in his, without speaking, because the Messenger of Allah (peace and blessings of Allah be upon him) commanded us to listen to the khutbah. He said, “If you say to your companion on Friday, ‘Listen!’ when the imaam is preaching, then you have engaged in idle speech.” (Narrated by al-Bukhaari, al-Jumu’ah, 882; Muslim, al-Jumu’ah, 1404. Agreed upon). 
He described his enjoining something good as being “idle speech” (laghw) at the time of the khutbah, so how about other kinds of speech? And the Prophet (peace and blessings of Allah be upon him) said, according to a saheeh hadeeth, “Whoever plays with the pebbles (on the ground) has engaged in idle speech (laghw).” (Narrated by Muslim, al-Jumu’ah, 1419) 
At Jumu’ah prayers, the believer should listen to the khutbah and be humble and solemn, he should beware of playing with pebbles on the ground or anything else. If someone greets him he should gesture to him without speaking. If he puts his hand in his when he offers his hand, without speaking, that is OK, as stated above. He should tell him after the khutbah that he should not have done that, and that which is prescribed if one comes in when the imaam is preaching is to pray two rak’ahs to “greet the mosque” and not to say salaams to anyone until after the khutbah has ended. If he sneezes he should praise Allah (say “al-hamdu Lillah”) to himself without speaking out loud.