Nedarim 85A

דרבי סבר: טובת הנאה ממון, ורבי יוסי בר רבי יהודה סבר: טובת הנאה אינה ממון?

That R' Yehuda HaNasi holds that the benefit of discretion is considered to have monetary value, and therefore a thief must pay the full value of the untithed produce. The owner has monetary rights in the priestly and Levitical gifts, by virtue of the fact that he may give his teruma and tithes to the priest and Levite of his choice. And R' Yosei, son of R' Yehuda, holds that the benefit of discretion is not considered to have monetary value, meaning that the owner of the produce has no monetary rights whatsoever in the teruma and tithes included in the untithed produce.

לא, דכולי עלמא טובת הנאה אינה ממון. אלא הכא במתנות שלא הורמו קא מיפלגי.

The Talmud rejects this explanation: No, as everyone agrees that the benefit of discretion is not considered to have monetary value. Rather, here they disagree with regard to whether priestly and Levitical gifts that have not yet been separated are considered as if they have already been separated, meaning the dispute is whether the untithed produce is categorized as a mixture of regular produce and tithes, or as a non-sacred category in and of itself. If they are not seen as having already been separated, the thief must restore everything he took. But if they are regarded as having already been separated, then the thief returns only the non-sacred portion of the produce, as the priestly and Levitical gifts did not belong to the owner.

ואי טובת הנאה אינה ממון ⁠— מה לי הורמו, מה לי לא הורמו?

The Talmud counters this argument: But if the benefit of discretion is not considered to have monetary value, what is the difference to me if the gifts have already been separated, and what is the difference to me if they have not yet been separated? Either way, the owner of the produce has no monetary rights in the portions of teruma and tithes contained in the untithed produce.