Beraisa: If a person said, "My house should acquire for me a hefker item that shall fall into it'', he has not said anything.

But if the item is already there, his words are valid,

This is like the sharecropper of R. Aba bar Mina who climbed a tree and he found chicks there and took them.

He asked Rav if he had acted correctly and Rav did not answer him.

He went and returned the chicks.

R. Aba bar Mina: Rav did not answer since the matter was unclear, but rather than returning them immediately, he should have kept them since he already acquired them when he found them.