A well-known argument in favour of the Pentateuch's divine authorship is that advanced (according to Artscroll's Stone Edition of the Pentateuch) by the Chatam Sofer, who says that the section of Leviticus dealing with the shemitah year "proves" (Artscroll's word) that God wrote the Torah, because a mere human would have to be a fool to promise what the text actually predicts: three years' worth of crop in the sixth year of the shemitah cycle. Since only God could make good on such a commitment, it must have been God who made it. While I agree with the Chatam Sofer's conclusion - that the Torah was divinely authored - I think that at least in our age, his argument suffers from major flaws. To wit:
1. Suppose Moses (not God) made the prediction. Suppose Moses knew he was going to die before the shemitah laws were to go into effect - in other words, before the Jews entered Israel - either because he was near death, or, quite possibly, because he simply had no intention of leading the Jews there. He could then promise whatever he wanted with impunity, knowing he would never have to answer for any promise's lack of fulfillment: that would be his successor's headache. Why would he make such a grandiose promise? Possibly to impress the people, as, indeed, the Chatam Sofer was impressed; possibly he had announced the shemitah year without duly considering what the nation would eat, and came up with the three-year-yield promise in response to challenges from the community. No doubt one could conceive of other reasons.
2. The argument assumes that the prediction was made by someone who believed that the Jews would keep the shemitah laws. If the prediction's originator thought otherwise, he could have made the promise confident that he would never be called to task for its non-fulfillment, since no one (or relatively few) would care even if the prediction did not come true. (Indeed, Rashi (Leviticus 26:35) indicates that the Jews observed shemitah less than half of the time between their entrance into Israel and the destruction of the First Temple.) The predictor could also argue, quite legitimately, that only when the nation was observing shemitah properly could it expect extra produce in the sixth year.
3. The argument assumes the predictor had not planned how to explain the lack of fulfillment of the prediction, even if the Jews did observe shemitah. He could always have resorted to the old stand-by that the Jews weren't righteous enough on the whole, observance of one specific commandment aside. A general indictment of a nation is very difficult to refute.
Why would the author of this portion of the Pentateuch have made the promise in the first place? Why would he have created a problem for himself, even if he had a strategy calculated to surmount it? See (1).
4. The argument assumes that the Pentateuchal passage in question was composed for the Jewish nation before or at the beginning of its tenure in Israel. Suppose (as many today might argue) that it was composed later. Let us discuss two cases:
Case 1: It is composed in Israel, but after the Jews have already been living there for some time, without the mitzvah of shemitah. The author invents the idea of the sabbatical year and the three-year-yield pledge, and claims that it had existed since the time of Moses. The people are not well-educated, and no one is the wiser. There is little or no risk to him: the entire nation is in violation, and has been so for a long time. Their practice will probably be very slow to change.
Why would he invent this commandment and associated promise? See (1); also, perhaps, to explain sufferings that have befallen his people ("None of you have been observing this law! No wonder there's a famine!") He could claim to have "found" it, as per the discovery of the Torah scroll in the Temple in the time of King Josiah (II Kings 22:8 ff).
Case 2: It is composed and presented to Jews in exile outside of Israel. The author need not worry; his composition is entirely theoretical (for his purposes), since he and his listeners are not in the land in which his prediction applies. The nation's non-observance while in Israel (which no one, of course, would contest) could be used to explain their exile.
In short, though I believe, for other reasons, that the Torah was divinely authored, it seems to me that many theories aside from divine authorship can account for the inclusion of the three-year-yield shemitah promise in the Pentateuch, and hence the existence of that promise sheds little or no light on the identity of its originator.