HE WHO EATS TERUMAH OF LEAVEN ON PASSOVER UNWITTINGLY, MUST REPAY [TO THE PRIEST] THE PRINCIPAL PLUS A FIFTH; IF DELIBERATELY, HE IS FREE FROM PAYMENT AND FROM [LIABILITY FOR] ITS VALUE AS FUEL.
This is based on the Torah commandment:
And if a man eats of the holy thing unwittingly, then he shall put its fifth part, and shall give it to the priest with the holy thing (Lev. 22: 14)
Which treats the eating of trumah as a theft from the priests and so there has to be restoration plus 1/5. But we’ve seen that since no benefit can be derived from chametz, it can’t have any value. How can you restore it?
R. Akiba holds: He must pay according to value; while R. Johanan b. Nuri holds: He must pay according to quantity.
Not everyone agrees on the non-existent value of chametz during Passover. But for those who do, you cannot deny its physical existence and therefore can be restored (plus 1/5th) in quantity.
Which gets us to an interesting point about quantity: what is the minimum amount considered “real” ?
Our Rabbis taught: He who eats as much as an olive of terumah must pay the principal plus a fifth. Abba Saul said: [He is not liable] unless it has the worth of a perutah (the smallest coin – let’s call it a penny).
What is the first Tanna's reason? — Scripture says: And if a man eat of the holy thing unwittingly and “eating” [requires] as much as an olive.
And Abba Saul: what is [his] reason? — Scripture says, and he shall give [unto the priest the holy thing] and “giving” is not less than the worth of a perutah.
So, is it the size or is the value? An olive or a penny?