Bava Metzia 66B

איבעית אימא: דאמר ליה קני מעכשיו.

If you wish, say instead that the Mishnah is referring to a case where the borrower said to the lender: If I do not repay you within 3 years, you will acquire the field from now, and they performed a formal act of acquisition. Since an act of acquisition was performed at the time, this is a proper sale and not an asmakhta.

אמרו ליה מר ינוקא ומר קשישא בני דרב חסדא לרב אשי, הכי אמרי נהרדעאי משמיה דרב נחמן: האי אסמכתא, בזמניה קניא, בלא זמניה לא קניא.

Mar Yanuka and Mar Kashisha, the younger and elder sons of Rav Ḥisda, said to Rav Ashi: This is what the rabbis of Neharde’a say in the name of Rav Naḥman: With regard to this asmakhta described in the Mishnah, it effects acquisition at its proper time, but it does not effect acquisition not at its proper time.

אמר להו: כל מידי, בזמניה קני, בלא זמניה לא קני.

Rav Ashi understood them to be saying that the lender acquires the field only when the loan is due, and he said to them: This statement seems to teach no novel ruling, as the same is true of every matter: It effects acquisition at its proper time, but it does not effect acquisition not at its proper time.

דלמא הכי קאמריתו: אשכחיה בגו זמניה ואמר ליה קני ⁠– קני, בתר זמניה ואמר ליה קני ⁠– לא קני. מאי טעמא ⁠– מחמת כיסופא הוא דקאמר ליה.

Rav Ashi suggests: Perhaps you meant to say this: If the borrower encountered the lender within its time, i.e., before the payment of the loan was due, and told him to acquire the field, the lender acquires the field, as it is assumed that the borrower was sincere. But if the borrower encountered the lender after its time, i.e., when the payment of the loan was due, and told the lender to acquire the field, he does not acquire it. What is the reason for this? The borrower says this to the lender out of shame with regard to his failure to pay the debt, but he did not really intend for him to acquire the field.