Reference for Bekhorot 57:10

חד זימנא תקינו ליה רבנן תרי זימני לא תקינו ליה רבנן:

why [does he take payment]? - The reason is that otherwise he might be suspected, and it might be said that the animal pronounced blemished is unblemished, and the reason he permits it is because he receives payment.

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