To begin with, Kinyan Gamur does not mean that the article belongs to him quite yet, even though Rabbi Yitzchak is referring to a Mashkon that the creditor receives after the loan has been made (and in fact, the debtor has failed to pay on time).

So what we are talking about is collateral that the creditor has in his domain, in order to force the debtor's hand at a later stage.

Rabbi Yitzchak learns his Din from the fact that the Torah writes in connection with the obligation to return a household article to the debtor each day (or night, according to the need), "and it shall be for a charitable act". It would not write this if the creditor had not acquired it legally. In effect, returning the article each day is a charitable act, which the Torah forces him to perform.

The question now arises: to what extent does he acquire it? According to Rashi, he acquires it to the point that he is liable even for Onsin. Otherwise, he maintains, it would be termed 'acquired'. This certainly goes nicely with the fact that a Malveh al ha'Mashkon (one who receives the security at the time of the loan, who does not acquire it) is considered a Shomer Sachar.

And that I take it, is the gist of your question. If, as Tosfos maintains, Rabbi Yitzchak considers the creditor to be only a Shomer Sachar, then what's the difference between him and a Malveh al ha'Mashkon? In other words, what does the Kinyan achieve? Incidentally, the Rambam and the Ra'avad indulge in the same argument as that of Rashi and Tos., in which case the same question will apply to the Rambam who explains Rabbi Yitzchak like Tosfos.

The answer, it seems to me, is as follows. I will refer to the Mishnah and its commentaries, because the Sugya in the Gemara is complex.

The Mishnah in the sixth Perek of Bava Metzi'a (Mishnah 7) declares a Malveh al ha'Mashkon a Shomer Sachar. The reason for this, the commentaries explain, is because of the Mitzvah he performs whilst looking after the object, which exempts him from giving a Perutah to a poor man, should he turn up whilst he is busy with the Mitzvah.

If not for the Mitzvah, it seems, the creditor would only be a Shomer Chinam. And that is what he ought to be in the case of Rabbi Yitzchak, seeing as the object belongs to him, in which case he has no Mitzvah per se to look after it. That is why the Gemara concludes that the very fact that the Torah places the article in his domain obligates him to accept liability on it, even though there is no Mitzvah.

Be'Virchas Kol Tuv

Eliezer Chrysler