The Poskim discuss whether a non-Jewish stockbroker is allowed to execute transactions for his Jewish client on Shabbat or Yom Tob. Clearly, the Jew is prohibited from explicitly instructing him to do so. The question is whether the Jew is allowed to tell him to buy or sell a certain stock when it reaches the desired price, even if it happens on Shabbat or Yom Tob.
Some Rabbis were lenient, because it is the price, not the Shabbat or Yom Tob causing the broker to act. However, Rav Moshe Feinstein (Russia-New York, 1895-1986) in his Iggerot Moshe, (OC Vol. 3:44) disagrees and considers such an arrangement to be a specific instruction to the broker to act on Shabbat or Yom Tob. Rav Moshe’s opinion cannot be discounted, and therefore it is prohibited to have such an arrangement with the stockbroker.
Nevertheless, if one informs the stockbroker that he will not be liable if he does not execute transactions on Shabbat or Yom Tob, such an arrangement is permitted. Even if the desired price was reached on Shabbat or Yom Tob, and the broker executed the transaction, the broker did so in his own interest to collect his commission. The Jew has already renounced any interest in transactions on Shabbat or Yom Tob.
It is prohibited to have a stockbroker execute transactions on Shabbat or Yom Tob, based on instructions regarding the price of a stock. The Jew must inform the broker that he will not be held liable if he does not buy or sell on Shabbat.