Is it permissible for a Muslim to greet a non-Muslim first?

A Muslim is not permitted to initiate the greeting of salam with non-Muslims, due to the prohibition of the Prophet (peace and blessings be upon him) against it. However, it is obligatory to return their greeting if they initiate it, by saying "Wa alaykum" if one is uncertain about their exact wording. As for greetings like "Ahlan wa sahlan" (welcome), it is not permissible to initiate them except out of necessity, such as to avert harm or to maintain kinship ties.