a) How does one reply to a salam which is written on a letter?
b) How do you reply to the salam which someone else has sent you (person A tells person B to give their salam to person C)?
In the name of Allah, Most Compassionate, Most Merciful,
a) Reply to the Salam verbally when you read the letter and also write it back if there is no fear of disrespect to the name of Allah Almighty.
b) You say: “Alayhi wa alayka assalam”, meaning: Peace and Salam be upon him (the sender) and also you.
And Allah Knows Best
[Mufti] Muhammad ibn Adam
Leicester , UK