Terumah More than a Sixth

The Gemora (Kesuvos 100a) asks: Why is this case (an agent undercharging for the property of an orphan) any different than that which we learned in the following Mishna: If one tells an agent, “Separate terumah for me (without specifying an amount), he should separate according to what the agent perceives is the mindset of the owner (either one-fortieth, one-fiftieth or one-sixtieth). If he cannot ascertain what the owner would want, he should separate one-fiftieth. If the agent has separated one in forty or one in sixty as terumah, the terumah is nevertheless is valid. (It is evident that although the agent has made a mistake, his actions are nevertheless valid?)

The Gemora answers: By the terumah, the agent has a valid excuse; he can say that he figured that the owner would separate terumah in a stingy manner or generously; however, in this case (where the agent charged too little for the property), the owner may tell the agent, “You should not have made a mistake.”

The Beis Yaakov asks: Isn’t the case of terumah a case where the agent erred in an amount which is more than a sixth; everyone would agree that the sale is invalid?

He answers: Since it is extremely common to err in this regard when separating terumah; even more than a sixth is regarded as having the same halacha as precisely a sixth.