What is the correct way to return the greeting of non-Muslims when they say the greeting of "salam" correctly?

If a disbeliever says, "Al-samu alaikum" (death be upon you), the reply is "Wa alaikum" (and upon you). However, if they clearly say "Al-salamu alaikum" (peace be upon you), then in fairness, the reply should be "Wa alaikum assalam" (and upon you be peace) or "Wa alaikum" (and upon you). It is not permissible for a Muslim to initiate the greeting of peace with non-Muslims, due to the prohibition of the Prophet (peace and blessings be upon him). But if they greet, they should be replied to in kind. So, if they say "Al-samu alaikum," one should say "Wa alaikum." And if they clearly articulate "Al-salamu alaikum," one should say "Alaikum assalam" or "Wa alaikum."