Commentary for Bava Metzia 9:8
הַמְקַבֵּל שָׂדֶה מֵחֲבֵרוֹ לְזָרְעָהּ שְׂעֹרִים, לֹא יִזְרָעֶנָּה חִטִּים, חִטִּים, יִזְרָעֶנָּה שְׂעֹרִים. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹסֵר. תְּבוּאָה, לֹא יִזְרָעֶנָּה קִטְנִית, קִטְנִית, יִזְרָעֶנָּה תְבוּאָה. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹסֵר:
If one received a field from his neighbor [in rental] to sow it with barley [i.e., for so much barley or wheat or money], he may not sow it with wheat; (If he received it to sow it with) wheat, he may sow it with barley, [for wheat weakens the soil more than barley.] R. Shimon b. Gamliel forbids it. [For it is harmful to the soil to sow it with one kind (of seed) one year, and with a different kind another. As to the halachah, all is according to the soil and the place. If the stipulation were for something that weakens the soil a little, he cannot change it and sow something that weakens it a lot. If the reverse, it is permitted.] (If he received it to sow it) with grain, he may not sow it with pulse; with pulse, he may sow it with grain. R. Shimon b. Gamliel forbids it.
Bartenura on Mishnah Bava Metzia
English Explanation of Mishnah Bava Metzia
If one leased a field from his fellow with the condition that he sow grain, he may not sow beans; [But if he leased the field with the condition that he sow] beans, he may sow grain. Rabban Gamaliel forbids this.
Both sections of this mishnah in essence teach the same law, so we will explain them together. If one leased a field with the stipulation that he sow a certain type of produce he may not sow something else that is more damaging to the soil of that field. He may, however, sow something that is less damaging to the soil. For instance, if he promised to sow barley he may not sow wheat since wheat is more damaging; but if he promised to sow wheat he may sow barley, since barley is less damaging. Similarly, if he promised to sow grain he may not sow beans which are more damaging; but if he promised to sow beans he may sow wheat which is less damaging. This is the anonymous opinion expressed in the first two clauses of each section. Rabban Gamaliel disagrees and states that one may not change the condition under any circumstance, even if the new seed will be less damaging to the field. Although there may not be exist a specific reason why in certain cases one should not change the conditions of the agreement, one is in general forbidden to do so.