According to the Torah (Vayikra 22:14) if someone accidentally eats teruma that is supposed to be given to the kohen, he needs to repay the value of the teruma to the kohen, plus an additional one-fifth of the value (20%) as a penalty. The Mishna (31b) teaches that if the teruma was hametz (i.e. it had been baked into bread), and it was accidentally eaten on Pesaḥ, the person who ate it needs to pay its value together with the penalty – even though hametz on Pesaḥ is ordinarily considered to have no value.
In explaining this law, the Gemara on our daf brings a baraita, where the tanna Kamma rules that someone who eats an olive-size amount of teruma needs to pay the value plus the penalty, while Abba Shaul rules that it is only true if the teruma that was eaten is worth at least a peruta (the smallest amount which still has monetary value). Each of the parties to this disagreement emphasizes a different part of the passage in Vayikra. The Tanna Kamma chooses to put emphasis on “And if a man eats a sacred item,” explaining that the term “eat” always means a minimum amount of food, which is traditionally understood to be a ka-zayit – food the size of an olive. Abba Shaul stresses the end of the pasuk – that the man must “give unto the priest the holy thing.” The term “give” means a significant amount – minimally the value of a peruta.
Rav Avraham Yitzchak Kook, in his book Tov Ra’ayah, suggests that the disagreement between Abba Shaul and the Hakhamim is based on two opinions in the Jerusalem Talmud. The Yerushalmi brings a discussion as to whether the source of the 20% penalty stems from the unique property of holiness of the teruma, which exists from the moment it is tithed by the farmer, or if it is a fine based on the fact that it was stolen from the kohen, which would not take effect until the kohen takes possession of the teruma. Thus, the Tanna Kamma understands giving the “holy thing” to the kohen as commanding that a minimum amount (a ka-zayit) must be given to the kohen; only then will the penalty be in place. Abba Shaul understands that the “holy thing” given is the penalty for misusing the teruma itself.