The Rambam, quoted in the Tur and Shulchan Aruch in Siman 295, holds that if something is "Matzuy B'shuk', the Shomer is Patur from a Shvua She Aino B'rshuso. It seems to me this is only a Ptur for when the Shomer chooses to pay, not when he swears to Patur himself. (We cannot apply the Svara that if a Shomer really wanted the Chefetz for himself, he'd just go buy it in the Shuk, if by swearing to Patur himself, he gets it for free!) But according to the Bach's understanding of the Rif, when a Shomer pays, he's already Patur from a Shvua She Aino B'Rshuso, because it's considered a Mechila by the Baal. So really the only time Matuzy B'shuk is a relevant Ptur according to the Bach's understanding of the Rif, is by cases of Hareini - where there's no Mechila, or by a case like Malveh Al Hamshkon, where the payment can't be considered a Mechila.
Daniel Steinberg, Columbus
Yes, according to the Bach's understanding of the Rambam, this is actually stated by the Rambam himself. The Bach (beginning of CM 295) writes that the Rambam agrees with the Rif. The Bach learns this from the fact that the Rambam (Hilchos She'elah u'Pikadon 6:1) begins by writing:
"A Shomer Chinam who said 'Hareini Meshalem and I will not swear'..., if the item is not Matzuy b'Shuk we are concerne that he has taken a liking to the object, so we make him take a Shevu'ah and afterwards he will pay."
We learn from this that it is only if he says "Hareini Meshalem" that we require him to swear, but if he actually pays, we say that the Mafkid was Mochel, which is what the Rif says too. It is Meduyak in the Rambam, who only mentions "Matzuy b'Shuk" after he writes "Hareini Meshalem," that if he actually paid, it does not matter whether or not it is Matzuy b'Shuk.
Kol Tuv,
Dovid Bloom
The Bach brings a proof to the Rambam's Chiddush Halacha of Matzuy B'Shuk from the Gemara in Eilu Metzios, that since Tefillin are so Matzuy B'Shuk, the Shomer Aveidah can sell the Tefillin and return the money to the Shomer. We see, the Bach says, that in such circumstances, i.e. Matzuy B'Shuk, we don't say that the Baal is particular to have his Chefetz returned to him.
I had a question on this proof:
Perhaps even if something is Matzuy, the Shomer would still have to swear Aino B'RShuso on it, because any Baal could always claim he wants his particular Chefetz back. But by Aveidah, when it's so Matzuy, we won't trouble the finder to be Mitapel with it, and that's why he can sell it right away. Meaning, by Aveidah, it's the fact that something is Schiach plus the fact that there's Tircha involved to the Shomer Aveidah - that we don't take into consideration the Baal's default Ratzon to have his particular Chefetz returned to him. But in regular, i.e. non-Shomrei Aveidah cases of Shomrim, the Matzuy factor by itself, without the Tircha consideration, i.e. is not enough of a reason to dismiss the Baal's wishes for his own Chefetz, and the Shomer would still have to swear Aino B'RShuso on it.
Reb Daniel, Baruch she'Kivanta to the Sevara of Rav Yitzchak Hutner zt'l!
1) This is from the Shi'urim "Reshimos Lev" that he gave in Yeshivas Rabeinu Chaim Berlin, on Perek ha'Mafkid, end of Siman 10:7 (page 197, DH v'Hi Ta'ama, and DH v'Kivan). He writes that if the Tircha that the Shomer Aveidah has in watching over the Aveidah is greater than the Tircha that the person who lost the Aveidah has to buy a new item, then we say that the Shomer is Patur and may sell the lost item.
2) Here is an attempt to defend the Bach against the challenge of the Pachad Yitzchak:
a) First, it should be pointed out that the Bach himself writes, "v'Hasam b'Sugya Medameh Din Motzei Aveidah l'Din Mafkid" -- the Sugya itself compares what one must do to watch over an Aveidah to what one must do to look after a Pikadon, not like Rav Hutner zt'l who seeks to make a distinction between the two.
(I think that the Bach means that, in Bava Metzia 29b, immediately after bringing the Din of Tefilin being very easy to buy, the Gemara cites the Din of somebody who borrows a Sefer Torah, which seemingly does not have its place in Perek Elu Metzi'os, which is a chapter that deals with lost and found. We learn from here that one equates the Halachos of Metzi'ah with the Halachos of Pikadon. -D.B.)
b) See also the Shulchan Aruch ha'Rav, Hilchos Metzi'ah u'Pikadon 37 (printed near the end of the Sefer) who writes that the Torah requires the finder of an object to go to a very great Tircha in order to return it.
Kol Tuv,
Dovid Bloom