Rav Shach asks on the Gra. I was at the sea and contemplating this question and it occurred to me a surprising thing that Rav Shach had a ready made answer to this question which he had just stated before! The issue is this: The Shulchan Aruch [Choshen Mishpat 241 paragraph 11 ]brings a "some say". The case is one says to two people write a document and by it take possession of my field. This alternate opinion hold he can not change his mind about the document. [The previous opinion was that he could].
The Gra in his notes there brings the source of this alternate opinion from Gitin [פרק התקבל]. That a woman can make a messenger to receive her divorce doc even though it has not be written yet.
The questions are these: a person can not cause possession [cause to be possessed or to possess] of something that has not yet entered into this world. And the source that the Gra brings for this does not seem to answer this difficulty since the woman is making a messenger which is an extension o her had. She is not now causing the divorce doc to be possessed.
The answer I believe starts with the observation that the messenger is not a general messenger but rather specifically for that divorce doc --which has not yet come into the world. But that is just the beginning of the answer. The total answer to the Gra comes from the very idea that Rav Shach brings in that law in אבי עזרי Avi Ezri Laws of Gitin perek 2 law 8 and 9. That is the case where a man says to two people to write a divorce doc and give it to his wife. In this case he is making them messengers on the total divorce process, not specially on the writing of the divorce. This is exactly the same thing in that opinion in the Shulchan Aruch Choshen Mishpat 243 and also in the law that the Gra brings. When one says write a doc and by it cause so and so to posses my field he is not making them specially agents for the writing but rather agents to cause possession. And that is the same thing with the law the Gra brings. She is not making her an agent to receive that specific doc but rather any doc that causes her to become divorced.
