Related for Bava Metzia 101:22

אמר לך רב אנא דאמרי אפי' לר' יהודה עד כאן לא קאמר ר' יהודה התם אלא דידעה וקא מחלה

I might think that the Divine Law did not apply the injunction of 'ye shall not defraud' to him. And had Scripture mentioned the vendee [only], that might be because he acquires [an article], for it is proverbial, 'When you buy, you gain'. But as for the vendor, who indeed loses thereby, as it is said, 'He who sells, loses,'<span class="x" onmousemove="('comment',' Money goes, and he who sells loses the article and probably the money too later on; but he who buys has a permanent gain — sentiments natural to a private individual as well as to a noncommercial, agricultural community. ');">¹⁹</span> I might think that the Divine Law did not exhort him, 'ye shall not defraud;' hence both are necessary. R. JUDAH SAID, THERE IS NO OVERREACHING FOR A MERCHANT. Because he is a merchant, has he no claim for overreaching? — Said R. Nahman in Rab's name: This was taught of a speculator.<span class="x" onmousemove="('comment',' So Jast. Rashi: a merchant who is a middleman, buying and selling from hand to hand. ');">²⁰</span> Why? Because he well knows the value of what he sells, but foregoes [part thereof] to him [the vendee], the reason that he sells thus [cheaply] being that he has chanced upon another purchase;<span class="x" onmousemove="('comment',' For which he needs immediate ready money. ');">²¹</span> nevertheless now he wishes to retract.<span class="x" onmousemove="('comment',' Possibly because his intended bargain did not mature. ');">²²</span> R. Ashi said: What is meant by 'THERE IS NO OVERREACHING FOR A MERCHANT? He is not subject to the law of overreaching. i.e., he can withdraw even for less than the [recoverable] standard of overreaching.<span class="x" onmousemove="('comment',' If he was deceived even by less than a sixth he can withdraw from the bargain, since that is his livelihood. ');">²³</span> It has been taught in accordance with R. Nahman: R. Judah said: There is no overreaching for a merchant, because he is an expert.<span class="x" onmousemove="('comment',' This proves that he has no redress, not, as R. Ashi said, that he is put in an advantageous position. ');">²⁴</span> HE WHO WAS DECEIVED HAS THE UPPER HAND. Who is the authority of our Mishnah, [seeing that] it is neither R. Nathan nor R. Judah ha-Nasi? For if R. Nathan — our Mishnah teaches, IF HE WISHES,<span class="x" onmousemove="('comment',' I.e., he has the choice of confirming the sale and recovering the fraud or cancelling the sale entirely. ');">²⁵</span> whereas the Baraitha<span class="x" onmousemove="('comment',' Supra 50b. ');">²⁶</span> does not state, If he wishes;<span class="x" onmousemove="('comment',' But only enables him to recover the Fraud but not cancel the transaction. ');">²⁷</span> whilst if it is R. Judah — our Mishnah refers to the Vendee [only],<span class="x" onmousemove="('comment',' As being able to cancel the sale, since it states, GIVE ME BACK MY MONEY. ');">²⁸</span> whereas the Baraitha refers to the Vendor.<span class="x" onmousemove="('comment',' V. supra 50b. ');">²⁹</span> (Mnemonic: <i>ZaB</i> RaSH.)<span class="x" onmousemove="('comment',' V. p. 398, n. 5. Z for EleaZar; B for RaBBah; R for Raba; R for ASHi. ');">³⁰</span> Said R. Eleazar: I do not know who taught this [Mishnah of] overreaching. Rabbah said: In truth, its authority is R. Nathan, but read in the Baraitha too, [If] he wishes [etc.]. Raba said: In truth, it is R. Judah ha-Nasi, but what the Mishnah omits is explained in the Baraitha.<span class="x" onmousemove="('comment',' V. p. 492. n. 2, and cf. p. 227. n 2. ');">³¹</span> Said R. Ashi: This too follows from the fact that it states. BOTH THE VENDEE AND THE VENDOR, yet proceeds to explain [the law of] the vendee [only]; this proves that the case of the vendor is merely left over. This proves it. It has been stated: If one says to his neighbour, 'I agree to this sale on condition that you have no claim of overreaching against me — Rab said: He [nevertheless] has a claim of overreaching against him. Whereas Samuel said: He has no claim of overreaching against him. Shall we say that Rab ruled in accordance with R. Meir, and Samuel in accordance with R. Judah? For it has been taught: If one says to a woman, 'Behold thou art betrothed<span class="x" onmousemove="('comment',' Lit., 'sanctified'. ');">³²</span> unto me on condition that thou hast no claims upon me of sustenance, raiment and conjugal rights' — she is betrothed, but the condition is null: this is R. Meir's view. But R. Judah said: In respect of civil matters, his condition is binding! — Rab can answer you: My ruling agrees even with R. Judah. R. Judah states his view there only in that case, because she knew [of her rights], and renounced them;

Tosefta Kiddushin