Commentary for Bekhorot 4:5

אין שוחטין על פיו – for perhaps because of reward he permits it.

If one takes payment for seeing first borns, they must not be slaughtered by his instructions unless he was an expert like Ila in Yavneh whom the sages permitted to accept four isars for small cattle and six as for large cattle, whether unblemished or blemished. Generally speaking it is forbidden for a sage to take wages in return for examining a first born to see if it is blemished or not. If a sage does take a wage and he declares it blemished the animal may not be slaughtered because it is feared that the wage acted as a bribe. The one exception is men who are like the legendary Ila in Yavneh. Ila was a great expert in blemishes and many priests would bring him their animals for examination. He was also wealthy enough that he didn’t need their payments. A bribe wouldn’t cause him to allow an unblemished animal to be slaughtered. Finally, he was known to be an honest man who would not lie in return for money. Because he exhibited all of these qualities, he was allowed to charge a fee for examining first borns. However, it is important to note that he collected his fee whether or not he declared the animal to be blemished. Obviously, it would be extremely problematic for him to collect his fee only if he declared it blemished. This would truly be a recipe for bribes.

כאילא – the name of a Sage that lived in Yavneh, and he was a pious individual and was not suspected of this, and whether he said to them that it (i.e., the firstling) was pure or whether he said to them that it had a blemish, he would take all of his salary.

ארבע איסרות בבהמה דקה ובגסה ששה – a large [animal] increases the trouble involved, to cast it to the ground and to turn it upside down and to check its blemishes. Therefore, its payment is greater.

בין תמים בין בעל מום – and even though that when he tells him that it is ritually pure, he does not benefit from him at all, and he takes his salary, for if he did not take his payment/reward , they wold come to suspect him that he said that it (i.e., the animal) has a blemish, and from here, Maimonides judges on those who slaughter [animals] for the public, for it is appropriate for them either that they should not take a salary at all, or that they should take a salary on the finding of [animals] that died of themselves or were torn, like those that are found in the field, similar to those who see the blemishes in firstlings.