Halakhah for Bava Metzia 7:11
כָּל הַמַּתְנֶה עַל מַה שֶּׁכָּתוּב בַּתּוֹרָה, תְּנָאוֹ בָטֵל. וְכָל תְּנַאי שֶׁיֵּשׁ מַעֲשֶׂה בִתְחִלָּתוֹ, תְּנָאוֹ בָטֵל. וְכָל שֶׁאֶפְשָׁר לוֹ לְקַיְּמוֹ בְסוֹפוֹ, וְהִתְנָה עָלָיו מִתְּחִלָּתוֹ, תְּנָאוֹ קַיָּם:
If one makes a condition contrary to what is written in the Torah, his condition is void. [This entire Mishnah is in accordance with R. Meir, who holds that if one makes a condition contrary to what is written in the Torah, even in respect to money matters, his condition is void. This is not the halachah. But, in monetary matters, even if one makes a condition contrary to what is written in the Torah, his condition stands.] And every condition preceded by an act is void. [If he prefaced the intended act to the desired condition, e.g., "This is yours, if you do this and this," (the condition is void). For it is not like the condition of the sons of Gad and the sons of Reuven, viz. (Numbers 32:29): "If they pass over … then you shall give, etc." where the condition precedes the act.] And whatever (condition) can be fulfilled at the end, if he stipulates it in the beginning, [the condition being prefaced to the act], the condition stands. [But if it is not possible to fulfill the condition, the condition is void and the act stands. For (in positing this impossible condition), he is only hyperbolizing, not really intending the condition, but only desiring to taunt and goad his neighbor with words.]