Bechorot 29 - Responsibilities of an Expert
If one who is not an expert nevertheless examined a firstborn in the possession of a kohen, found a permanent blemish, and it was slaughtered based on his authorization, the animal is buried and the "expert" pays for the damage.
In general, if one adjudicated a monetary case, or a question of ritual purity, and caused a monetary damage by his incorrect decision, he needs to make restitution from his own property. However, if the judge was regarded a legal expert by the official court and they authorized him, then he does not have to make a restitution, because he is similar to an authorized slaughterer who bungled the slaughter.
If one takes a payment to inspect firstborns, his rulings are void, unless he is an exceptionally saintly person and takes a standard fee regardless of the outcome.
One should not take a fee for judging, testifying, or sprinkling the waters of a red heifer, and if he does, his actions are void. Just as God taught Torah to Moses for free , so one should teach others for free. However, he can be compensated for the wages lost by taking off from work, assuming he is paid as a laborer.
Art: Carl Neuman - Wise Words