Ruling on returning the greeting to a disbeliever

Is it obligatory to return the greeting to a disbeliever, and does the one who leaves it off commit a sin?
Returning the greeting to a disbeliever is obligatory, based on the saying of Allah the Exalted: [Surat al-Nisa: 86] This is the wisdom, and Allah knows best.Because Allah said: And He did not say: "When some of you greet others," nor did He say: "When your brothers greet you."So he said: Therefore, the response is obligatory. But if the one who greets says, "Welcome so-and-so" or "So-and-so is welcome," is the response obligatory? The apparent meaning of the Noble Qur'an is that the response is obligatory; because this is called a greeting by custom, even though the best greeting is to say: "Peace be upon you, and the mercy of Allah and His blessings." However, since people have become accustomed to saying "Welcome," "So-and-so is welcome," or "May Allah greet you" as a greeting, the apparent meaning of the Noble Qur'an requires that you respond to him.