Is it obligatory to return a greeting in writing to one who sent it in writing, or is it sufficient to return it verbally and then answer his question?

It is obligatory to return the greeting of peace (salaam) due to the generality of the Almighty's saying: (And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner]). If a greeting of peace is sent with a person, it must be returned immediately verbally. If the greeting is written, it is permissible to return it either verbally or in writing. Ibn Hajar Al-Haytami mentioned that it is commendable to return the greeting to the messenger as well, by saying: "And upon you and upon him be peace," because the person present deserves more consideration. The obligation of returning the greeting verbally, even in the absence of the one who sent the greeting, preserves the right of Allah and the right of the human, and because the messenger conveys the reply to its sender. As for replying in writing, its wisdom is clear, for the letter reaches the one who sent the greeting.