Comentario sobre Baba Metziá 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:
Si uno recibió un campo de su vecino [en alquiler] para sembrarlo con cebada [es decir, por tanta cebada, trigo o dinero], no puede sembrarlo con trigo; (Si lo recibió para sembrarlo) trigo, puede sembrarlo con cebada, [porque el trigo debilita el suelo más que la cebada.] R. Shimon b. Gamliel lo prohíbe. [Porque es perjudicial para el suelo sembrarlo con un tipo (de semilla) un año y con otro tipo diferente. En cuanto a la halajá, todo está de acuerdo con el suelo y el lugar. Si la estipulación fuera por algo que debilita un poco el suelo, no puede cambiarlo y sembrar algo que lo debilite mucho. Si es al revés, está permitido.] (Si lo recibió para sembrarlo) con grano, no puede sembrarlo con pulso; con pulso, puede sembrarlo con grano. R. Shimon b. Gamliel lo prohíbe.
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.