Gemara

Question (Rava): If Reuven lent Shimon in a coinage, and they increased (the weight of) the coins, what is the law?

Answer (Rav Chisda): He must return coins that are accepted now, even if the new coins are much bigger.

Question: These coins can buy more food! (This is Ribis!)

Answer #1 (Rav Ashi): If they buy more because they are bigger, he returns only coins with the same purchasing power. If the price of food declined, he returns the same monetary amount, even though it can buy more.

Question: The new coins have more weight!

Answer #2: A case occurred, and Rav Papa and Rav Huna brei d'Rav Yehoshua checked with a moneychanger, and found that eight new coins equaled 10 old coins (that were lent).

Rishonim

Rif and Rosh (9: 12): The old coins were eight Perutos each, and the new coins were 10 each. If the price of food did not decline due to this, he returns the same number of coins. Even though the new coins weigh another fifth (of the total weight, which is a q4 of the original weight) more, we are not concerned, since they do not buy more food. If the price of food declined, even a little, he deducts from the new coins accordingly. If the new coins are more than a fifth heavier, he returns the same amount (of weight) as the old coins, especially if food became cheaper. The same applies if they make the coins smaller.

Rosh: We are not concerned for returning up to a fifth more, for letter of the law one should return a currency that is accepted at the time, if not that it looks like Ribis if there is more metal. Since only another fifth is added, this is not a gain regarding the metal. It would cost this much to melt it down! The Ra'avad says that this is only if the coins became bigger, for it looks like Ribis, but not if the new coins are smaller (then he always returns the same number, even though they weigh less). Presumably, the Rif is correct (the same applies if they make the coins smaller), for we must be concerned for the lender's loss.

Rashba (4:287): Up to a fifth one does not deduct, for even in the initial currency, some coins were up to a fifth bigger. Alternatively, if one would melt the new coins, the fire would consume a fifth, or it would cost a fifth to make new coins. Also the Ra'avad said so.

Note: Why should the cost of making new coins (other than what is consumed by the fire) be a fifth of the value? The cost to make new gold coins should be the same as to make new silver or copper coins!

Question: If he would melt the old coins, he would also lose a a fifth (of their weight). Therefore, he gained!

Answer #1 (Drishah YD 165:2): He had no need to melt the old coins; he could spend them. The new coins do not buy more; the only advantage of them is if he will melt them.

Rebuttal (and Answer #2 - Taz 1): They are worth more if he melts them to make a Kli! Rather, we are not concerned for gains that come only through destroying the old coins. Had the lender bought food, he could have sod it now to get bigger coins! The Gemara's question was that the borrower can say 'I will make for you old coins from the new ones, and keep the extra metal.'

Mordechai (110): Rashi explained that 'lent in a coinage' means that he gave merchandise on credit, but one who lends gets back only what he gave. At the end of his life, he retracted, and wrote in a Teshuvah that it refers to a loan of coins on condition to pay back coins, and not other merchandise. The same applies to one who sold on condition to pay a certain coinage, or obligated himself to pay a Kesuvah in a particular coinage. If food did not get cheaper due to the increase in the coins, and it was at most a fifth, he does not deduct. One is not particular about this, and the borrower expected to pay bigger coins if they will change.

Rambam (Hilchos Malveh 4:11): If Reuven lent Shimon in a coinage, or one wrote a Kesuvah to his wife in a known coin and they increased the weight of the coins:

If food became cheaper due to the increase, even any amount, he deducts the amount of the increase;

If food did not get cheaper due to the increase, he does not deduct. He pays with coins that are currently accepted.

This is when the increase is at most a fifth, e.g. the coins weighed four and they made them five. If the increase was more than a fifth, he deducts the amount of the increase, even if food did not get cheaper. The same applies to one who lent in a coinage and they made the coins smaller.

Magid Mishneh: Rashi and the Rashba say that if there is a place where the old coins are accepted like before, and the lender goes there, he pays the old coins. This is no worse than coins that were disqualified, and they are still accepted in another place.

Poskim

Shulchan Aruch (YD 165:1): If Reuven lent Shimon in a coinage, and they increased the weight of the coins: If food became cheaper due to the increase, even any amount, he deducts the amount of the increase. If food did not get cheaper due to the increase, he does not deduct. He pays with coins that are currently accepted. This is when the increase is at most a fifth, e.g. the coins weighed four and they made them five. If the increase was more than a fifth, he deducts the amount of the increase, even if food did not get cheaper.

Beis Yosef (DH Im): If the increase was more than a fifth, or food got cheaper due to the increase, and the lender took the full increased amount, Sefer ha'Terumos says that some say that this is Ribis Ketzuzah, and others say that it is Avak Ribis, and he need not return it. The Rambam holds that it is Avak Ribis, for he holds that even gvg extra l'Shem Ribis, if there was no stipulation from the beginning, is only Avak Ribis.

Taz (2): If it is more than a fifth, he deducts the full increase, like we find regarding one who sold Peros and more than the tolerance for other species or dirt was mixed in (CM 229:1).

Shulchan Aruch (ibid): The same applies to one who lent in a coinage and they made the coins smaller.

Gra (2): The Poskim disagree with the Ra'avad. It is unreasonable that the borrower pay back less than he borrowed!

Rema: If they did not increase the coins and food became cheaper, he pays with new coins. If they changed the coins and it is not known whether they are a fifth bigger or smaller, we rely on Nochri experts in the field who spoke Lefi Tumo (unaware of the consequences), or courts appointed over this.

Source (Sefer ha'Terumos, cited in Beis Yosef DH Kosav): The courts would not (lie, and) ruin their reputation for what they were appointed over. We ask two Nochrim not in front of each other, like we do for a document written in Persian (when only Nochrim understand it).

Rema (ibid): If the king commanded that anyone who pays must use the new coins, we follow this, for the law of the kingdom is binding. It is not a problem of Ribis or theft.

Beis Yosef (DH uk'Inyan): The Ramban says that it is not a problem of Ribis, for the borrower does not pay willingly. If not for the king's decree, it would be theft!