What challenge was the astrologer made to face by his last client of the day?

At he end of the day, a stranger appears. The astrologer lures him over, and the man, a stranger, makes a bet with him. The stranger wants to know the answer to one question..... if the astrologer cannot answer his question, he wants his money back (with interest). The astrologer then counters that if he is right, the stranger must pay him triple his normal fee, to which the stranger again counter that if he's wrong, the astrolger must pay him double, his triple fee. Yes..... it's a bit confusing.