Related for Bava Metzia 101:20
איתמר האומר לחבירו על מנת שאין לך עלי אונאה רב אמר יש לו עליו אונאה ושמואל אמר אין לו עליו אונאה לימא רב דאמר כרבי מאיר ושמואל דאמר כרבי יהודה
only if the purchaser was defrauded; how do I know it if the vendor was overreached? Because Scripture states,'…acquirest…ye shall not deceive' — Now, both vendee and vendor must be written, for had the Divine Law stated [the law only of] the vendor — that is because he knows his purchase;<span class="x" onmousemove="('comment',' Hence, if he overreaches, he does it wantonly. and therefore the overcharge is returnable. ');"><sup>17</sup></span>
Tosefta Kiddushin
"[Be betrothed to me] with the understanding that if I die you will not bound to a levir"—she is betrothed but his stipulation is invalid, for he stipulated against what was written in the Torah, and anyone who stipulates against what is written in the Torah, his stipulation is invalid. "With the understanding that I will have no responsibility for you for clothing or sex"—she is betrothed but his stipulation is invalid. This is the pneumonic: Anyone who stipulates against what is written in the Torah regarding a monetary matter—his stipulation stands; with a non-monetary matter—his stipulation is invalid.
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