Responsa for Bava Batra 252:9

אמר רב יוסף אמר איש פלוני בני בכורי הוא נוטל פי שנים איש פלוני בכור הוא אינו נוטל פי שנים דלמא בוכרא דאמא קאמר

<b><i>GEMARA</i></b>. [Must] it be said [that] our Mishnah<span class="x" onmousemove="('comment',' Which forbids any stipulation that is contrary to a law of the Torah. ');">²⁴</span> is not in accordance with R. Judah? For, if [it be suggested that it is in accordance with] R. Judah. surely he said, [it may be asked]. [that] in money matters one's stipulation is valid'.<span class="x" onmousemove="('comment',' Even if it is contrary to a law of the Torah Since our Mishnah deals with money matters and yet it is stated that one's stipulation that is contrary to the Torah, is invalid, it obviously cannot agree with R. Judah's view. ');">²⁵</span> For it was taught: If a man said to a woman, 'Behold thou art consecrated unto me<span class="x" onmousemove="('comment',' The formula of marriage used by the bridegroom is, 'Behold, thou art consecrated unto me by this ring according to the law of Moses and Israel'. ');">²⁶</span>

Teshuvot Maharam