Terumos 6:5-6

Terumos 6:5

If a person accidentally ate terumah, he may not repay the kohein using leket, shich’cha or peah (the various portions designated for the needy), nor from hefker (produce declared ownerless), first tithe from which terumah has been taken, nor from second tithe or sanctified produce that has been redeemed. This is because one holy item cannot take the place of another; this is the opinion of Rabbi Meir. The Sages allow one to repay with these things.

Terumos 6:6

Rabbi Eliezer says that one may repay terumah with a different species of produce so long as he pays with a species that is superior to the one he ate; Rabbi Akiva says that one may only repay with the same species. Therefore, if a person ate terumah cucumbers of sixth-year produce, he must wait for the cucumber crop of the eighth year (after the sabbatical) to be able to repay. They each base their ruling on the same verse: “he shall give the holy item to the kohein” (Leviticus 22:14). Rabbi Eliezer understands this to mean anything that can be made holy, while Rabbi Akiva understands it to mean the same holy thing that was eaten.
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