If the owner already told them to write, sign, and give over the Shtar, then
why do they have to ask him again before giving over the Shtar? If the person
wants to change his mind, he would come and tell us that he has changed his
mind! Why do we have to suspect that he changed his mind?
(a) The ROSH (5:4) explains that since the person did not make a Kinyan
Chalifin to transfer ownership of the property immediately, but rather asked
that a Shtar be written to transfer ownership at the time of the delivery of
the Shtar, it shows that he is reluctant to part with his property. Had he
made a Kinyan Chalifin through the witnesses, the witnesses would have
acquired the property on behalf of the recipient and it would have belonged
to the recipient immediately. Instead, he told the witnesses to write a Shtar
and to give the Shtar to the recipient in order to give him the land. Since
the owner demonstrated reluctance in parting with his land, the witnesses
must confirm with him before giving over the Shtar and finalizing the Kinyan.
This might be the intention of TOSFOS (DH Kesuvu) as well.
(b) The RITVA, quoting the RA'AVAD (see Ra'avad in Hilchos Malveh v'Loveh
11:1), explains the Gemara differently than Rashi. The Gemara is not
discussing someone who is giving land to his friend with a Shtar. Rather, the
Gemara is discussing a husband who is writing to his wife a Tosefes for her
Kesuvah. The husband is saying to witnesses at the time of Erusin that they
should write the Tosefes in the Kesuvah at the time of Nesu'in. Since he does
not tell the witnesses to write the Tosefes to his wife right now, at the
time of Erusin, he shows that he is reluctant to give away his land to her as
a Tosefes to her Kesuvah, and therefore they must ask him for his consent
again at the time of Nesu'in. If he obligates himself to give his wife the
Tosefes immediately at the time of Eirusin, by having the witnesses make a
Kinyan on the Tosefes on behalf of the wife, then the husband does not have
to be consulted again before writing the Tosefes in the Kesuvah and
delivering it to his wife.
According to this explanation, it is clear why the Gemara records this case
in this Sugya which discusses various aspects of the Kesuvah. (The GE'ONIM,
in a Teshuvah cited by the Shitah Mekubetzes, give a similar explanation.)
(c) The RAMBAM (Hilchos Malveh v'Loveh 11:1) explains that the Halachah
recorded in the Gemara refers exclusively to a borrower who is giving a Shtar
Chov, a voucher for a loan, to his friend. There is a significant difference
between a loan made without a Shtar and a loan made with one. When one
borrows money without giving a Shtar to the lender, then if the lender
demands repayment of the loan *after* the borrower paid it back, the borrower
can simply reply that it was paid and he does not need to prove it. On the
other hand, when one borrows money and gives a Shtar to the lender, if the
lender wants to collect the money using the Shtar after the borrower paid it
back, the borrower will have to bring proof that he already paid back the
loan.
It is possible that according to the Rambam, we require that the borrower
confirm his request to deliver the Shtar because he might not have taken into
account all of the ramifications of this act. He might not have considered
that if he gives a Shtar to the lender he will have to prove that the loan
was repaid. Therefore, the witnesses must ask his consent again before giving
over the Shtar.
The Rambam explains that when the Gemara says that if a Kinyan was made with
the witnesses, they do not have to ask him again before giving over the
Shtar, it means as follows. If the borrower made a Kinyan with the witnesses
to *obligate himself to give his friend a certain sum of money*, then it is
assumed that he wants them to write it down -- for a Kinyan performed before
witnesses is presumably done with intention to have it written down (like
Rashi says).
(d) The HAGAHOS MAIMONIYOS (Hilchos Malveh v'Loveh 11:1, #2) writes in the
name of RABEINU TAM that the Halachah of our Gemara applies whenever a person
gives his friend a gift. Basing himself on the Yerushalmi, he says that if a
person asks a Shali'ach to give a gift to someone else, the Shali'ach must
ask for the giver's confirmation before giving the gift, in order to make
sure that the giver was serious (since he is giving something away without
receiving any compensation for it).