Beitzah 35A

והלא מותרו חוזר, ושמעינן ליה לרבי אליעזר דאמר: כל היכא דמותרו חוזר ⁠— לא קבע!

The Talmud challenges: How can one say that the very decision to partake of the fruit establishes it as fixed with regard to tithes? But isn’t it true that even if one declared his intention to eat it the following day, it can nevertheless be assumed that the remaining fruit is restored to the pile? And we have heard that R' Eliezer explicitly said: Anywhere that its leftovers are restored, it is not established with regard to liability for tithes at all.

דתנן: הנוטל זיתים מן המעטן ⁠— טובל אחת אחת במלח ואוכל. ואם טבל ונתן לפניו עשרה ⁠— חייב. רבי אליעזר אומר: מן המעטן טהור ⁠— חייב, מן המעטן טמא ⁠— פטור, מפני שהוא מחזיר את המותר.

This is as we learned in a Mishnah in tractate Ma’asrot: One who removes olives from a vat [ma’atan] where they are temporarily stored before being pressed may dip them one by one in salt and eat without tithing them first, since he is eating them one at a time. Although he is eating them with salt, it is not considered a fixed meal. And if he dipped and placed several olives before him, such as 10, they are liable in tithes. However, R' Eliezer says: One who eats from a ritually pure vat is liable to separate tithes; one who eats from a ritually impure vat is exempt because he returns the surplus to the vat.

והוינן בה: מאי שנא רישא ומאי שנא סיפא? ואמר רבי אבהו: רישא במעטן טהור וגברא טמא, דלא מצי מהדר ליה.

And we discussed it: What is different in the first clause of the Mishnah and what is different in the latter clause; why is the issue of purity relevant to this case? And R' Abbahu said: The first case is referring to a ritually pure vat and a ritually impure person, who transfers his impurity to the olives he touches. He may not return the olives to the vat because he would thereby render all the remaining olives ritually impure. Therefore, from the outset he takes only the amount he wishes to eat. This is enough to consider it a fixed meal, and he must tithe them.