1) PROOF FOR REBBI'S EARLIER OPINION
QUESTION: The Gemara (44a) teaches that in his youth, Rebbi maintained that gold was considered currency with regard to silver and, therefore, a person who performs an act of acquisition on silver creates a Kinyan and obligates the buyer to pay gold. Later, Rebbi retracted this view and ruled that the opposite is true: silver is considered currency and gold is considered merchandise. The wording of the Mishnah reflects Rebbi's final ruling (even though the wording of the Mishnah in the Yerushalmi reflects his earlier ruling; indeed, the name of the Perek there is "ha'Kesef" and not "ha'Zahav").
All of the Rishonim rule like Rebbi's final decision, and in accordance with the text of the Mishnah. Although the Gemara (44b) concludes that there is proof from a Beraisa that gold is considered currency, nevertheless it is clear from a number of sources in the Gemara that the Gemara follows the view of Rebbi in his later years and considers gold to be merchandise in relation to silver.
The Gemara cites a Mishnah (Ma'aser Sheni 2:7) in which Beis Shamai and Beis Hillel argue as to whether one may redeem silver coins of Ma'aser Sheni with gold coins. Beis Shamai forbids it and Beis Hillel allows it. The Gemara explains this Mishnah in three different ways. According to all three explanations, Beis Hillel views gold as merchandise in relation to silver, as Beis Hillel permits one to redeem silver coins of Ma'aser Sheni with gold coins even though Ma'aser Sheni may be redeemed only with currency (outside of Yerushalayim) and not with Peros (merchandise). The view of Beis Hillel (whose view is almost always accepted as the Halachah) in this regard clearly conflicts with the ruling of all of the Poskim. Moreover, the could have cited Beis Hillel as proof for the ruling of Rebbi in his youth. Why is the opinion of Beis Hillel not a clear proof for the ruling of Rebbi in his youth, that silver is considered merchandise and gold is considered currency?
(a) The ROSH (4:1, and in TOSFOS HA'ROSH) writes that according to Rebbi's later ruling, there is no such Mishnah in Ma'aser Sheni; Beis Shamai and Beis Hillel never argued over this point. According to Rebbi's later ruling, even Beis Hillel agrees that one cannot redeem silver coins of Ma'aser Sheni with gold coins.
The RASHBA and TOSFOS RABEINU PERETZ write that just as Rebbi reversed the phrase in the Mishnah here in his later years, he also reversed the opinions in the Mishnah in Ma'aser Sheni. Thus, Rebbi later maintained that it was Beis Hillel who prohibited redeeming silver with gold, and Beis Shamai who permitted it. (According to this opinion, it is not clear why this Mishnah was not included in the list of Chumros of Beis Hillel in the fourth chapter of Eduyos.)
(b) The PNEI YEHOSHUA proposes that the opinion of Beis Hillel as it appears in the Mishnah can be reconciled with Rebbi's later opinion, that silver is considered currency. Rebbi's later opinion maintains that silver is considered currency in the case of a purchase because silver is used more frequently as currency. Although gold is more important, the power to effect a Kinyan is determined by what is used more as a currency. In contrast, with regard to Ma'aser Sheni the primary factor that determines whether one object can be redeemed with another is the importance of the coin, not its ability to be used as currency. A coin made from a material that is more important can be used to redeem a coin of lesser importance. That is why Beis Hillel permits redeeming silver coins with gold coins.
The BA'AL HA'ME'OR uses similar logic to reconcile the Mishnah in Ma'aser Sheni with Rebbi's later opinion. (The Ba'al ha'Me'or writes that Beis Hillel is consistent with Rebbi's later opinion even if Beis Hillel permits one to redeem fruit with gold coins. Although gold is considered a currency with regard to Ma'aser Sheni, it is not considered a currency with regard to a purchase. The Pnei Yehoshua, however, argues that if Beis Hillel allows gold to be used to redeem fruit of Ma'aser Sheni, then he indeed argues with Rebbi's later ruling that gold is considered merchandise in relation to silver.)
(c) TOSFOS RABEINU PERETZ (also cited by the Ritva) makes a similar distinction between a purchase and the redemption of Ma'aser Sheni. He writes that even if gold is considered merchandise, the Chachamim permitted one to redeem Ma'aser Sheni with gold coins because they are easier to carry to Yerushalayim (since one gold coin is equivalent to several silver coins). (This is more easily understood according to the opinions of the Rishonim here who say that mid'Oraisa one is permitted to redeem Ma'aser Sheni with any non-consumable item. According to these Rishonim, the Torah prohibits redeeming Ma'aser Sheni with fruits only because they will spoil before the owner arrives in Yerushalayim. The requirement to redeem Ma'aser Sheni only with coins was instituted by the Chachamim to prevent a person from redeeming Ma'aser Sheni with fruit (see Ritva). Since that requirement itself is an enactment of the Chachamim, they provided an exception to their rule with regard to gold coins in order to facilitate carrying the Ma'aser Sheni to Yerushalayim.)
The Ritva explains that this is true only according to the opinion that maintains that Beis Shamai and Beis Hillel argue even with regard to redeeming fruit with gold coins. Although Beis Shamai prohibits redeeming fruit with gold coins, this is not because he equates the Halachos of redeeming Ma'aser Sheni with the Halachos of a purchase. Both Beis Shamai and Beis Hillel agree that gold is considered currency in relation to fruit, and therefore one should be permitted to redeem fruit with gold coins. Beis Shamai prohibits such a redemption because of the stringency of the Chachamim which was applied to Ma'aser Sheni. Just as Beis Shamai distinguishes between the laws of Ma'aser Sheni and the laws of a purchase by being stringent with regard to Ma'aser Sheni, Beis Hillel also distinguishes between them -- by being lenient with regard to Ma'aser Sheni.
(d) The RAMBAN writes that according to the last two explanations in the Gemara for the dispute between Beis Shamai and Beis Hillel, Beis Hillel permits redeeming silver coins of Ma'aser Sheni with gold coins because of the verse that teaches that one may redeem with "Kesef Sheni," a second set of coins to redeem the first. The Ramban explains that this verse teaches that not only may one redeem a coin of Ma'aser Sheni with a coin that is a more liquid currency, but one may even redeem a coin of Ma'aser Sheni on any coin, even one that is less liquid. Thus, although with regard to a purchase gold is considered merchandise in relation to silver, Beis Hillel still permits one to use gold to redeem a silver coin of Ma'aser Sheni, since gold has the status of a coin at least with regard to purchasing fruits. As long as it is considered a type of coin, the requirement of the verse is fulfilled.
2) USING COINS FOR "KINYAN CHALIPIN" BUT NOT FOR "KINYAN KESEF"
QUESTION: The Gemara discusses whether coins may be used to effect a Kinyan Chalipin.
It is clear from the Mishnah that coins cannot be used to acquire movable objects with a Kinyan Kesef. (According to Reish Lakish, the Torah recognizes only Meshichah, and not Kesef, as a valid Kinyan. According to Rebbi Yochanan, the Torah recognizes Kesef as a Kinyan but the Rabanan annulled it and instituted Meshichah in its place, as the Gemara will explain (46b).)
If coins may be used to effect a Kinyan Chalipin, what determines whether the coins were intended to be used for a Kinyan Chalipin (and thus they do acquire movable objects) or for a Kinyan Kesef (and thus they cannot acquire movable objects)? After all, both Kinyanim are done in the exact same manner. What is the difference between them?
(a) RASHI here (and in the end of the Mishnah) implies that the type of Kinyan one makes is determined by the wording he uses. If one explicitly states that he is giving the money to accomplish a Kinyan Chalipin, the money will be Koneh (if coins can accomplish Chalipin). If he does not make this statement, the money will not be Koneh. This also appears to be the opinion of the RIF.
(b) TOSFOS (46a, DH Shema Mina) and the TOSFOS HA'ROSH (ibid.) first explain like Rashi, and then they add that if the recipient of the coins immediately returns them, this act also may indicate that the coins were given for a Kinyan Chalipin. His act of returning the coins implies that the coins were like a Sudar, a cloth which is used for Chalipin and which is given to the seller only as a "Matanah Al Menas l'Hachzir," with the intent that it accomplish the Kinyan and then be returned (see Nedarim 48b).
(c) The TOSFOS HA'ROSH (46a) cites the RIVAM who offers another distinction between the two Kinyanim. The Rivam writes that coins can be considered Chalipin only when two conditions are fulfilled: the buyer mentions the Kinyan as he gives the coins to the seller (for example, he says, "I am giving this amount of money in order to acquire this object"), and both the coins and the merchandise are present. In contrast, if the seller quotes a price at one point, and the buyer later hands over money without specifying why the money is being given, the transaction is considered Kinyan Kesef, which cannot be used to acquire movable objects.
(The Tosfos ha'Rosh objects to this explanation.)
3) A MINTED COIN WILL EVENTUALLY BECOME INVALIDATED
QUESTION: The Gemara teaches that coins cannot be used to accomplish a Kinyan Chalipin, because "Tzurta Avida d'Batla" -- one who receives a coin "has in mind the impression that was stamped on the coin, and the impression will eventually be invalidated" (when a new king comes to power and requires a new stamp, invalidating the old one).
How does this intention of the recipient of the coin explain why a coin cannot be used for Chalipin? Why does it matter if the impression will eventually become invalidated, if presently it is legal tender?
(a) RASHI (DH Mishum) explains that since a coin can become invalidated by a new king, it is considered an "incomplete" item, a "Davar she'Eino Mesuyam," which may not be used to effect a Kinyan Chalipin. It is comparable to a slice of fruit, which cannot be used for Chalipin even according to those who normally permit the use of fruit to accomplish a Kinyan Chalipin.
The RAMBAN points out that this logic explains only why a coin cannot be used to effect a Kinyan Chalipin, but not why a coin cannot be acquired through a Kinyan Chalipin (for example, when one hands over a utensil in order to acquire another person's coins); no law states that only "whole" objects can be acquired through Chalipin. The Gemara later, however, concludes that coins cannot be acquired through Chalipin just as they cannot accomplish Chalipin.
(The MACHANEH EFRAIM (Kinyan Chalipin #3) suggests that Rashi may agree with the TESHUVOS HA'RASHBA (1:1104), who writes that one cannot even acquire a "Davar she'Eino Mesuyam" through Chalipin.)
(b) The RAMBAN and RITVA explain that the Gemara means that Chalipin can be accomplished only with an object that has value in its own right, and not simply because of international convention. Coins are not viewed by the buyer and seller as slabs of silver and gold, but as acceptable currency. Currency has no inherent value (like today's paper bills); as soon as a king changes the mintage, the old coins lose their value as currency. Therefore, coins cannot be used to accomplish a Kinyan Chalipin. They are similar to Shtaros, which the Gemara considers "Ein Gufan Mamon" (Bava Kama 117b) -- items that have no inherent value but are attributed a value due to what is written in them (Ramban).
This point is derived from the fact that the Torah describes Chalipin as performed with a "shoe" (see Gemara 47a). Just as a shoe has inherent value, any object used for Chalipin must have inherent value (and just as the field which was purchased with the shoe has inherent value, any object that is purchased with Chalipin must have inherent value) (Ritva).
Alternatively, this point may be derived from logic alone. People do not depend on something as "inconstant" as a coin, and therefore they do not attach to it enough importance for it to accomplish, or be acquired with, a Kinyan Chalipin (CHINUCH, Mitzvah #336; see also CHIDUSHEI HA'RITVA Yeshanos here).
The Ramban adds that Rashi may agree with this analysis. Perhaps Rashi explains the Gemara differently only because Rav Papa is the author of the Gemara's statement that coins cannot be used for Chalipin, and Rav Papa himself says (shortly afterwards in the Gemara) that although coins cannot be used to accomplish Chalipin, they can be acquired through Chalipin. According to that view, Rashi's explanation is valid. However, according to the view (to which Rav Papa himself eventually subscribes) that coins cannot even be acquired through Chalipin, it is clear that coins are not just a "Davar she'Eino Mesuyam." Rather, they are considered to have no inherent value, and therefore they can neither effect Chalipin nor be acquired through Chalipin.