Bekhorot 29B

ורב יהודה בר אמי משמיה דרב יהודה אמר: בית הפרס שנידש טהור.

And Rav Yehuda bar Ami says in the name of Rav Yehuda: A beit haperas that has been trodden underfoot, thereby creating a path, is pure, and one no longer needs to be concerned about bones, as the entire prohibition is a stringency by rabbinic law. Nevertheless, a priest who walks through such a field may not partake of teruma until he has purified himself.

אי נמי: בשאר טומאות, דלא מזהר עלייהו.

Alternatively, the Talmud suggests that the Mishnah is referring to a case where the priest becomes impure with other forms of impurity, with regard to which priests are not admonished. A priest is prohibited from becoming impure only through contact with a corpse. Yet, he may not partake of teruma if he becomes impure even in any other manner.

אם היה זקן ⁠— מרכיבו על החמור. תנא: כפועל בטל. אמר אביי: כפועל בטל של אותה מלאכה.

§ The Mishnah teaches: If the one examining the firstborn, or the judge, or the witness, was an elderly person, the one who required his services transports him on a donkey. And in all these cases, although it is prohibited to take wages, the one who required his services gives him his wages like the wages of a laborer, as he was unable to perform his usual labor that day. A baraita states (Tosefta, Bava Metzia 4:11): The owner gives him his wage as if he were an idle laborer. Abaye says: It means that he is paid as a laborer who is idle from that typical labor of his from which he is kept idle. In other words, he must receive the amount of money a person would be willing to accept to refrain from his current occupation and engage in the actions discussed in the Mishnah. This calculation accounts for both the degree of difficulty of his steady employment and the amount of his remuneration.