Gemara

44b: Rebbi taught to his son R. Shimon 'gold acquires silver.'

In his youth, he taught that silver acquires gold. He reasoned that gold is more important, therefore it is the currency. In his old age, he reasoned that silver is more accepted (by sellers), so it is the currency.

(Rav Ashi): Presumably, his first reason is correct, for the Mishnah says 'copper acquires silver, but silver does not acquire copper. If silver acquires gold, this teaches that even though silver is the commodity with respect to gold, it is currency with respect to copper;

However, if silver is the currency with respect to gold (even though gold is more important), all the more so it is currency with respect to copper (which is less important and less accepted by sellers than silver)!

Rejection: One might have thought that in places where copper is used, it is more accepted than silver, and it is the currency. The Mishnah teaches that this is not so, since there are places where it is not accepted.

Rav borrowed gold coins from R. Chiya's daughter. Their value increased (with respect to silver). R. Chiya ruled that he returns coins like he borrowed.

Inference: Gold is the currency. If gold were the commodity, this would be like lending a Se'ah (a measure) of grain on condition to return a Se'ah, which is forbidden (lest grain rise in value. This is Ribis mid'Rabanan!)

Rejection: Rav had gold coins at home. In such a case, one may lend Se'ah b'Se'ah.

(R. Yochanan): One may not lend a Dinar for a Dinar.

He does not forbid lending a silver Dinar for a silver Dinar. Surely, all agree that silver is a currency with respect to itself (and permit this)! Rather, he forbids lending a gold Dinar for a gold Dinar. Since regarding commerce gold is the commodity (with respect to silver), we consider its value to fluctuate, therefore also regarding loans, one may not lend it Dinar for Dinar.

75a (Mishnah): One may not say 'lend to me a Kor of wheat on condition that I will return a Kor after the harvest'. He may ask for a loan until his son comes or until he finds the key.

(Mishnah): Hillel forbids lending a loaf until setting a price for it, lest wheat rise in price and the borrower will return (a loaf worth more, which is) Ribis.

(Rav Yehudah): Chachamim permit Stam.

Rishonim

Rif: Since gold is the commodity with respect to silver, one may not lend a (gold) Dinar for (to pay back) a Dinar, lest he lend a Dinar worth 24 silver Dinarim, and at the time of payment it will be worth 25. We cannot say that silver decreased in value, for it is the currency! Rather, gold decreased (Hagahos Chavos Ya'ir - increased). Therefore, one may not lend a Dinar for a Dinar, just like one may not lend Se'ah b'Se'ah. Tana'im argue about which of gold and silver is the currency, and there was no explicit ruling. R. Yochanan holds that gold is the commodity, like Rebbi said in his old age, and no Amora argues.

Nimukei Yosef (DH Gemara): Since gold is currency with respect to Peros, Se'ah b'Se'ah should be permitted. It is a mere stringency of Ribis to forbid it, since it is the commodity with respect to silver.

Rosh (4:1): We hold like Rebbi's retraction in his old age, like our Stam Mishnah. Rav Ashi said that what he said in his youth is more reasonable, but this was rejected. R. Yochanan holds like Rebbi's old age, and forbids lending a Dinar for a Dinar. We must say that this refers to a gold Dinar.

Terumas ha'Deshen (54): People lend gold for gold. The Ge'onim ruled that gold is the commodity, but Tosfos disagrees. Also, nowadays most people have Metaltelim worth gold, and if they wanted to sell them cheaply, if they could not sell them in this city, they could sell them elsewhere. This is like Se'ah b'Se'ah when the borrower has Mezumanim. Metaltelim worth coins are as good as coins. Even if he has only one gold coin, he may borrow many, like Se'ah b'Se'ah, other Metaltelim do not help, only coins, for Peros are normally sold only for coins. Metaltelim are sold for gold coins. Also, the Rosh says that silver is the currency with respect to gold because it is Charif (sellers accept it) and important. Nowadays, almost all our (silver) coins are mostly copper, and they are not important compared to gold, so gold is not the commodity. In the days of the Ge'onim, of Tosfos, and even 100 years ago (i.e. in the middle of the 14th century) silver coins were very good and mostly silver. Perhaps people were stringent then (not to lend gold for gold).

Poskim

Shulchan Aruch (YD 162:1): One may not lend Se'ah b'Se'ah. This is even if he did not fix a payment date.

Beis Yosef (DH Bein): If this were permitted when a date was fixed, the Gemara would have rejected the proof from Rav's loan by saying that a date was fixed. Even though the Mishnah forbids a loan 'until after the harvest', it merely gives a typical case. Alternatively, it is for parallel structure with the Seifa, which permits 'until my son comes or until I find the key.'

Shulchan Aruch (ibid): The same applies to everything, except for silver coins (currently) accepted for currency.

Beis Yosef (DH v'Chen): All silver coins are permitted. Copper coins are forbidden, even though they are the currency regarding Metaltelim, for they are considered the commodity with respect to silver. We forbid lending gold coins for this reason, even though they are the currency with respect to Metaltelim. The Rif, Rosh and Nimukei Yosef say so.

Shach (3): Even silver coins are forbidden if they are not accepted now, e.g. they were disqualified, even if they used to be accepted.

Shulchan Aruch (ibid): We are concerned lest the commodity rise in price, and he pays back more than was lent. We permit only if a price is fixed; if the commodity will rise in value, he wil repay this amount. If one borrowed without fixing a price and the commodity rose in value, he pays back its value at the time of the loan. If the price decreased, he pays back a Se'ah, like he borrowed.

Rema: Some say that nowadays gold coins are like silver and one may lend gold for gold. The custom is to be lenient. One should not protest, for they have whom to rely on.

Beis Yosef (DH ha'Malveh): If in the city there is a market with a fixed price for gold Dinarim, and at any time one can buy and pay back, one may lend a gold Dinar for a gold Dinar (Sefer ha'Terumos citing the Rif). Terumas ha'Deshen permits even if there is no fixed price and the borrower has no gold Dinarim, as long as he has Metaltelim. Since he could sell them and get gold coins, it is as if he has gold coins. He also permits if silver coins in the city are poor, and are mostly copper.

Shach (4): In the days of the Gemara, silver coins were good. Nowadays they are mostly copper, so gold is the currency.

Taz (2): This is because nowadays most people own things worth gold, and they could sell them cheaply if they wanted.

Rebuttal (R. Akiva Eiger): If there is a fixed price for Peros, Se'ah b'Se'ah is permitted even if he has no coins. If there is no fixed price, it is forbidden even if he has coins. The same applies to lending coins! The Shach's reason is valid.

Beis Lechem Yehudah: In Polin, people lend copper for copper. Granted, according to the Taz, people could sell Metaltelim to get copper coins. However, according to the Shach this is forbidden! Since it is a Safek, the borrower should make sure to have a few such coins. Beis Hillel (the Posek) says that here, the number of Shillings to buy a silver or gold coin fluctuates, (but) we can say that there was a fixed rate at the time of the loan.

Note: The Terumas ha'Deshen and Rema permitted gold coins. They did not mention a Heter for copper, even though presumably, people owned Metaltelim that they could sell for copper coins!

Beis Yosef (DH v'Zeh): The Rashba says that if a gold Dinar is spendable, surely one may lend it to pay back the same. It did not rise in price, rather, Peros declined. The Gemara said that surely, one may lend silver Dinar for Dinar, since it is currency with respect to itself. Even if coins are not accepted for payment, if there is a steady fixed price and one can buy them at any time, it is permitted. It is no worse than Peros in storehouses and boats (once they reach the market, one may contract for other Peros). Perhaps gold coins are better, for they are currency with respect to Peros. We permit Peros with a steady fixed price. All the more so Dinarim are permitted!

Shulchan Aruch (2): One may borrow Se'ah b'Se'ah if he has (even) a small amount of what he borrows.

Taz (3): Rav did not ask to borrow a Dinar for a Dinar. He merely asked to borrow Dinarim. R. Chiya's daughter did not know that Rav had Dinarim. Therefore, Rav thought that he may not return Dinarim, for they rose in value. R. Chiya answered that it is as if his Dinarim were acquired to her. This is a leniency of Se'ah b'Se'ah.