Bekhorot 36B

דאית ליה לאשתמוטי.

as he has the easy option of extricating himself from the accusation that he did not fulfill the condition of the one who appointed him, by claiming that in his opinion the individual from whom he purchased the produce was to be trusted. Since the issue of the third party’s trustworthiness is subjective, the agent is not afraid to lie.

סיפא ודאי מסייעא ליה: מאיש פלוני ⁠— הרי זה נאמן.

The Talmud suggests: The latter clause of this Mishnah certainly supports the ruling of Rav, that people do not lie with regard to any matter that will be revealed. How so? The latter clause states that if the one instructed the agent to purchase produce specifically from so-and-so, who is reliable with regard to tithes, then this agent is deemed credible to claim that he adhered to the instructions. Since the one who appointed him mentioned a specific individual, the agent is not suspected of lying, as that individual can be subsequently questioned.

התם, כיון דאית ליה תובע, מירתת.

The Talmud rejects this proof: There, in the Mishnah, since the agent has a claimant, he is afraid to lie. In other words, since the one who appointed him specified an individual, the agent presumes that he intends to verify the matter, and therefore he will not risk lying. By contrast, in Rav’s case, since it was the priest who volunteered the information that a specific Israelite gave him an already blemished firstborn animal, he is not concerned that the examiner might seek out the Israelite to confirm the actual facts.

אמר רבי ירמיה בר אבא: מנא ליה ליהודה הא? אנא בגידול קבעתיה, וגידול קבע בדידיה. והכי אמרי ליה: נאמן ישראל לומר ״בכור זה נתתי לכהן במומו״.

Upon hearing Rav Yehuda’s statement in the name of Rav, R' Yirmeya bar Abba said: From where does Yehuda know this halakha? Actually, I taught this halakha to Giddul in Rav’s name and Giddul, in turn, taught it to R' Yehuda. But Giddul corrupted my statement, as this is how I said it to him: An Israelite is deemed credible to say: I gave this firstborn animal to a priest with its blemish already inflicted upon it, and it may be slaughtered and eaten.