Pesachim, chapter two, mishnah 4. This mishnah deals with the law of a non-kohen who eats Terumah that is chametz on Pesach. To understand the mishnah, we first need to review the rules of payment for a non-kohen who ate Terumah.
Payments of a non-kohen who ate Terumah:
A non-kohen who ate Terumah unintentionally must repay the principal - the amount he ate - plus a fifth. That added fifth is twenty-five percent of the principal, since adding twenty-five to one hundred gives one hundred twenty-five, and the addition is a fifth of the total. This payment is called keren v'chomesh, principal and a fifth.
It is worth being precise here: when a person eats Terumah unintentionally, the payment is not determined by the monetary value of the Terumah, but by the quantity he ate. If he ate a gallon of Terumah, he must repay a gallon of that same type of fruit or produce plus a fifth, and it makes no difference whether the value went up or down. Moreover, what he gives the kohen itself takes on the status of Terumah.
All of this applies to eating unintentionally. But if he ate intentionally, meaning he knew it was Terumah and ate it, the payment is determined by monetary value, and there is also no penalty of an added fifth.
Chametz on Pesach - devoid of monetary value:
This distinction is what decides our case, since chametz on Pesach has no monetary value at all. Chametz is among the things from which benefit is forbidden, and there is no way to derive any benefit from it, so it loses all of its monetary worth.
The words of the mishnah:
"Ha'ochel terumas chametz b'Pesach" - one who eats Terumah that is chametz, and eats it during the days of Pesach.
"B'shogeg meshalem keren v'chomesh" - even though the chametz has no monetary value, he must pay the principal plus a fifth. The reason: payment for eating Terumah unintentionally is not based on its having monetary value, but is an obligation the Torah imposes according to the quantity that was eaten.
"B'meizid patur mitashlumin" - one who eats intentionally, knowing that it is Terumah, pays according to monetary value, and since what he ate had no monetary value at all - it is chametz, after all - he is exempt from paying.
He is even exempt from paying its value as firewood: Terumah that became tamei may be used as fuel for burning things, and therefore it has some value. But no benefit at all can be derived from chametz, so it may not even be used as fuel. Since it has no value whatsoever, he is not obligated to pay even its worth as firewood.
In summary: in this mishnah we learned that one who unintentionally eats Terumah that is chametz on Pesach is obligated in principal and a fifth, because payments for an unintentional act are determined by the quantity eaten and not by monetary value. One who eats intentionally, however, whose payments are determined by monetary value, is exempt from payment - and even from its value as firewood - since chametz on Pesach is among the things from which benefit is forbidden and it has no value at all.