What should we reply if a disbeliever says "peace be upon you" to us?

It is not permissible for a Muslim to initiate greeting a disbeliever with "peace" (Salam). If a disbeliever says "Al-Sam Alaikum" (a curse, meaning 'death be upon you') or does not clearly articulate the greeting, one should respond with "Wa Alaikum" (and upon you). However, if a Muslim is certain that the disbeliever said "Salam Alaikum" (peace be upon you) using a legitimate wording, the majority of scholars are of the opinion that it is obligatory to return the greeting with "Wa Alaikum Al-Salam" (and upon you be peace) out of justice, due to the generality of Allah 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]).