What should a Muslim do if a non-Muslim greets him with the Islamic greeting?

The majority of scholars are of the opinion that responding to the greeting of a disbeliever should be with the words: "Wa 'alayk" (And upon you) or "Wa 'alaykum" (And upon you all), based on the Prophet's (peace be upon him) saying: "When the People of the Book greet you with 'As-salamu alaykum,' then say: 'Wa 'alaykum'." Another group has held the view that it is permissible to respond to him with the same greeting used for a Muslim.