Is it permissible to say "wa al-rahmah" (and mercy) when returning a greeting of peace, and to do so consistently?

One must return the greeting in the same form as the one who offered it, or better, as stated in the Almighty's words: (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]). The obligatory return is to be equivalent, while an increase is recommended. The "better" return encompasses the wording, voice, and cheerfulness. So, if you are greeted with "As-salamu alaykum," it is permissible to respond with "Wa alaykum as-salam wa rahmatullah," as it is equivalent or an increase. If "As-salamu alaykum wa rahmatullah" is said, the response "Wa alaykum as-salam wa rahmatullah" is considered equivalent, but it is better to adhere to the prophetic formula: "As-salamu alaykum wa rahmatullahi wa barakatuh."