What is the ruling on one who returns a greeting with "Salam" only, and one who returns a greeting with "Al-Salamu wa Rahmatu Allahi wa Barakatuh"?

The best form of greeting is "Assalamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah), and the response is "Wa alaykum assalam wa rahmatullahi wa barakatuh" (And upon you be peace and the mercy of Allah and His blessings). It is permissible to initiate the greeting by saying "Salam alaykum" (Peace be upon you) without the conjunction "wa" (and) or the definite article. Some scholars have held that it is permissible to suffice with "Assalam" (The Peace) or "Salam" (Peace) for both initiating and responding to the greeting, citing as evidence the saying of Abraham, peace be upon him, to the angels: "Salam."