From the rulings concerning giving salaam
by The Albaani Site
Question: When someone says, “As-Salaamu alaikum,” do we add, ‘… wa barakaatuhu …’ when replying to him?
Answer: Yes. There is nothing to stop you saying up to wa barakaatuhu. And the order is not a condition, i.e., when someone says to you, “As-Salaamu alaikum,” you have the option of saying, “Wa alaikum salaam wa rahmatullaah” or “Wa alaikum salaam wa rahmatullaahi wa barakaatuhu.”
I (Shaikh Ali Hasan) asked: As for saying, “… wa maghfiratuhu …” [after wa barakaa-tuhu] then it is not said unless the person says upto wa barakaatuhu?
Shaikh al-Albaani: Yes.
Shaikh Ali Hasan said in a footnote to the above, “And whoever reports that the Shaikh retracted from his stance about the correctness of the addition, “… wa maghfiratuhu …” when returning the greeting of salaam then he is mistaken! Yes, the Shaikh retracted from his statement that the chain of the narration was saheeh li-dhaatihi but he declared it to be hasan due to its supporting narrations, so pay heed to that, may Allaah have mercy upon you. [And that is] just as I read, in his own handwriting, may Allaah have mercy upon him, in his notes to the third volume of Silsilah as-Saheehah, hadith number 1449.”
Su’aalaat of Shaikh Ali Hasan al-Halabi, vol. 2, p. 567, no. 393