Is it permissible to engage a person in conversation who begins talking to you prior to extending the Islamic greeting i.e., "As-Salam `Alaykum [Peace be upon you]"?
(Part No. 24; Page No. 123
You should exchange conversation with him then instruct him that the Sunnah is to first greet the Muslim by saying: "As-Salam `Alaykum", then begin talking to him.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.