Question: I heard that Ha-Rav Shlomo Goren permitted using the Heter Mechirah and then later forbade it?
Answer: Ha-Rav Ha-Gaon Shlomo Goren's opinion is that after the establishment of the State of Israel we can no longer utilize the Heter Mechirah since even though an individual can sell his field to a non-Jew, there still remains a sort of "super-ownership" by the entire State. As a result, there is still Jewish ownership by the State of Israel. When Rav Goren served as the Chief Rabbi of Israel, the Halachah nonetheless followed the majority of the Rabbis on the Rabbinical Council of the Chief Rabbinate and the Heter Mechirah was indeed utilized. His personal opinion, however, was that it cannot be used (article in the magazine "Be-Machanayim" of the Military Rabbinate in the year 5619. Torat Ha-Medinah p. 445. Torat Ha-Moadim p. 624).
And there is another great Rabbi who agrees: The Satmar Rebbe. Ha-Rav Yoel Teitelbaum had many arguments against the Heter Mechirah. He argued that even according to the opinion that the Heter Mechirah is halachically acceptable (which he did not uphold), it no longer applies after the establishment of the State of Israel on account of the "super-ownership" of the State (Shut Divrei Yoel #96-98 and printed in a separate booklet entitled "Shalosh Teshuvot").
The difference, however, is that Rav Goren held this opinion because the State of Israel is a great and holy Mitzvah, while the Satmar Rebbe reasoned as he did because he saw the State as impure, and a rebellion against Hashem.
Rabbenu Ha-Rav Tzvi Yehudah however rejected the proofs of Rav Goren, and argued that the sale of land sold by an individual is considered a valid sale regardless if there is Jewish sovereignty over Eretz Yisrael or not. Therefore, the Heter Mechirah still applies even after the establishment of the State of Israel (Techumin Volume 7, p. 23).