Does a divorce take place or is expiation obligatory for one who said: "Divorce is upon me, if I saw him in this tunnel with fire burning him, I would not ask about him," then he returned the greeting to this person and fulfilled his needs?

If you are unsure whether you said "Upon me be divorce" or "Upon me be divorce by three [talāqs]", the default is to assume no number was specified. Swearing an oath of divorce falls under the ruling of conditional divorce. According to the majority of scholars, divorce occurs when the condition is met. However, Shaykh al-Islam Ibn Taymiyyah held the view that an expiation for an oath (kaffārat al-yamīn) is incumbent if the one swearing the oath did not intend divorce. If the shaykh who issued the fatwa to you is trustworthy, then you may act upon his fatwa that an expiation for an oath is due. One must be cautious of repeating the question and moving between muftis without necessity, and also be cautious of being lenient in uttering divorce, and avoid making an oath a barrier to doing good. Allah (the Most High) says: "And do not make [your oath by] Allah an obstacle against your being righteous, pious, and making reconciliation among the people." The Prophet (peace be upon him) said: "By Allah, if Allah wills, I will not swear an oath and then see something else as better than it, except that I will expiate for my oath and do that which is better."