Bava Batra 157B

אלא הא מני ⁠– רבי מאיר היא, דאמר: אדם מקנה דבר שלא בא לעולם.

Rather, in accordance with whose opinion is this Mishnah? This Mishnah is in accordance with the opinion of R' Meir, who says: A person can transfer ownership of an entity that has not yet come into the world. Similarly, R' Meir maintains that one can place a lien on property that the borrower will subsequently acquire.

אמר רב יעקב מנהר פקוד משמיה דרבינא, תא שמע: שטרי חוב המוקדמין ⁠– פסולין, והמאוחרין ⁠– כשרין.

Rav Yaakov from Nehar Pekod says in the name of Ravina: Come and hear proof from a Mishnah (Shevi’it 10:5): Promissory notes that are antedated, i.e., that are dated prior to the date on which the loan actually was given, are invalid. This is because the promissory note places a lien on the borrower’s property. By dating the document earlier than the loan itself, the lender appears to have a lien on property that the borrower sold prior to taking out the loan, enabling the lender to fraudulently repossess it from the buyer. But promissory notes that are postdated are valid, as this does not enable the lender to defraud a buyer.

ואי סלקא דעתך ״דאיקני״⁠ – קנה ומכר, ״דאיקני״⁠ – קנה והוריש, לא משתעבד; מאוחרין אמאי כשרין? ״דאיקני״ הוא!

The Talmud explains: And if it enters your mind to say that property that the borrower acquires after receiving the loan is not liened even when he writes: The property that I will acquire should be liened, and he acquires property and sells it to others, or when he writes: The property that I will acquire should be liened, and he acquires property and bequeaths it to his heirs, why, then, are postdated promissory notes valid? They should be invalid, as in some instances they enable the creditor to fraudulently repossess property that is not liened, e.g., if the borrower acquires property after receiving the loan but before the date on the promissory note, and he sells it after that date. This case is comparable to one where the borrower writes: The property that I will acquire should be liened.