Bekhorot 29A

כאילא ביבנה, שהתירו לו (ביבנה) חכמים להיות נוטל ארבע איסרות לבהמה דקה, וששה לגסה, בין תם ובין בעל מום.

like Ila in Yavne, whom the rabbis in Yavne permitted to take a wage of 4 issar for issuing a ruling concerning a small animal and 6 issar for issuing a ruling concerning a large animal. They permitted this provided that he would be paid whether it turned out that the firstborn was unblemished or whether it was blemished.

גמ׳ מאי טעמא? האי נפיש טירחיה, והאי לא נפיש טירחיה.

Talmud: The Talmud asks: What is the reason that the rabbis in Yavne said that Ila could take a wage of 4 issar for issuing a ruling concerning a small animal but 6 issar for issuing a ruling concerning a large animal? The Talmud answers: Examining this one, a firstborn of a large animal, for blemishes involves great effort; but examining that one, the firstborn of a small animal, does not involve such great effort.

בין תם ובין בעל מום. בשלמא בעל מום ⁠— משום דקא שרי ליה, אלא תם אמאי? דאם כן, אתי למיחשדיה, ואמרי: האי בעל מום תם הוא, והאי דקא שרי ליה ⁠— משום אגרא.

§ The Mishnah teaches that one like Ila may take his wages provided that he would be paid whether it turns out that the firstborn is unblemished or whether it is blemished. The Talmud inquires: Granted, if the animal is blemished he takes his wages, because he deems the animal permitted to be slaughtered and eaten; but if he examined an unblemished animal, why does he take his wages? The Talmud answers that if so, i.e., if he would be paid for examining only a blemished animal, people would come to suspect him and say that this animal that he declared to be blemished is in fact unblemished, and he declared it blemished and thereby deemed it permitted to be eaten only due to the fact that he wanted to claim his wages.