Bekhorot 35B

מימר אמר: לא שביק צורבא מרבנן ויהיב לדידי. ״רועי כהנים״ והן ישראל ⁠— אינן נאמנין, דחיישינן ללגימא.

The reason is that it is assumed that the priest-shepherd would say: My Israelite employer will not forsake a priest who is a Torah scholar and give the blemished animal to me, an unlearned shepherd. The case of priest-shepherds is where the shepherds are Israelites in the employ of a priest. These shepherds are not deemed credible to testify, as we are concerned that they might be lying for a swallow of the firstborn that their priest employer would give them in exchange.

וכל שכן כהן לכהן, דחיישינן לגומלין, וחיישינן ללגימא. ואתא רבן שמעון למימר: נאמן על של חבירו, ואינו נאמן על של עצמו. ואתא רבי מאיר למימר: החשוד על הדבר, לא דנו ולא מעידו.

And all the more so the testimony of a priest for the sake of another priest is not deemed credible, as we are concerned that the two might engage in reciprocal behavior, and we are concerned that the priest-shepherd might be lying for a swallow of the firstborn offering. And Rabban Shimon ben Gamliel comes to say that the priest-shepherd is deemed credible to testify with regard to the firstborn offering of another priest, but is not deemed credible to testify with regard to a firstborn belonging to himself. And R' Meir comes to say that a priest who is suspect about the matter of causing a blemish may neither adjudicate nor testify in cases involving that matter, even on behalf of another.

בשלמא למאן דאמר: רועי ישראל והן כהנים נאמנין ⁠— היינו דאתא רבי מאיר למימר: החשוד על הדבר לא דנו ולא מעידו.

The Talmud raises a difficulty: Granted, according to the second opinion, i.e., the one who says the first tanna maintains that the case of Israelite shepherds is where the shepherds are priests in the employ of an Israelite, and they are deemed credible to testify, this is the reason that R' Meir comes to disagree and say that a priest who is suspect about the matter of causing a blemish may neither adjudicate nor testify even for the sake of an Israelite employer.