Nedarim 43B

אבל אם היה הפקירו קודם לנדרו ⁠— הרי זה מותר. ואי אמרת עד דאתי לרשות זוכה, מה לי נדרו קודם להפקירו, מה לי הפקירו קודם לנדרו?

However, if his declaration that the food is ownerless predates his vow, it is permitted for the other person to eat the food. And if you say that the reason for the opinion of R' Yosei is because the food remains in the possession of the owner until it comes into the possession of the one who acquires it, what is different for me if his vow predates his declaration that the food is ownerless, and what is different for me if his declaration that the food is ownerless predates his vow? In any event, the item remains in the possession of its owner and the one for whom benefit is forbidden benefits from it.

הוא מותיב לה והוא משני לה: כל הנודר אין דעתו על מה שהפקיר.

The Talmud answers: R' Abba raised the objection and he answered that objection. The difference between the cases does not relate to the halakhot of ownerless property; rather, it relates to the nature of vows. The intent of anyone who vows is that the vow not apply to an item that he rendered ownerless. Therefore, when he declares the food ownerless and then vows, he does not intend to include the ownerless food in his vow, and the prohibition does not take effect upon it.

מתיב רבא: מקצתן לראשון וכולן לשני ⁠— ראשון קנה,

Rava raised an objection: With regard to a person on his deathbed who ordered his executor to distribute all his property, if he recovers, he may retract the gift. However, if he ordered him to distribute only a portion of his property and kept the rest, he cannot retract the gift. If he ordered him to distribute his property to two people and said: A portion of the property is given to the first person and all of the remaining property is given to the second person, then if he recovered, the first person acquired the property that was given him, as it was a partial gift.