Menachos, Chapter 13, Mishnah 4. This mishnah also continues to deal with donations that were not clearly specified, and with the question of how much a person must bring in order to fulfill his obligation.
One who pledges gold:
One who says "Harei alai zahav" - "I take upon myself gold" - has obligated himself to bring a gold coin. The term "gold" is ambiguous: he may mean a gold coin, or he may mean a lump of gold, an ingot of raw material. The correct way to understand the mishnah is that it is speaking of a gold coin, and in the end we will return to discuss the case where he meant an ingot.
The ruling is that he must bring no less than a gold dinar - he brings the smallest gold coin in circulation, which is the gold dinar. We do not assume that he intended more than that, since he himself did not have anything specific in mind, and since this is a gold coin, that is enough.
One who pledges silver:
One who says "Harei alai kesef" - "I take upon myself silver" - has obligated himself to bring a silver coin to the Beis HaMikdash. Here too he did not name a specific sum or the size of the coin, but simply intended a silver coin in general. Therefore he brings the smallest silver coin in circulation in his place and in his world, and no less than a silver dinar.
Someone familiar with talmudic weights and measures will raise a difficulty: the Mishnah and Gemara mention many silver coins smaller than the dinar, which is a relatively large coin:
The maah - one sixth of a dinar.
The pundyon - one twelfth of a dinar.
The issar - one twenty fourth of a dinar.
The Gemara asks this question and answers: the context of the mishnah is the place where it was stated, and there the silver dinar really was the smallest of the coins. There were larger coins there, such as the shekel and the sela, but the dinar was the smallest of them. A comparison: someone who says today in America "a silver coin" means a silver dollar, which is the smallest silver coin in use and in circulation today.
One who pledges copper:
One who says "Harei alai nechoshes" - "I take upon myself copper" - has obligated himself to bring a copper coin to the Beis HaMikdash. Copper coins come in every size, down to truly tiny perutos, the size of a small fingernail. For the sake of consistency we should have obligated him in the smallest copper coin, that is, a perutah; but the Gemara says that it is not reasonable that he meant a perutah.
A comparison: a person who says he is giving a coin as a gift does not have a penny in mind, whose value is utterly negligible (and they have even stopped minting it). So too here: it is not reasonable that he intended a tiny, nearly worthless perutah, but rather a larger quantity of copper. Therefore the mishnah says that the value of the copper coin he brings must be no less than a silver maah, which is one sixth of a dinar.
One who obligated himself in a sum and does not remember its amount:
If the one who made the vow says that he did not have any coin in mind at all, but rather a certain sum of money - whether in silver, in gold, or in copper - and he did have a specific amount in mind at the moment he obligated himself, but he no longer remembers how much it was, this is a serious problem. The Mishnah says that he must bring up to the amount about which he can say with certainty: I definitely did not obligate myself in more than this.
A parable for this: someone bidding on aliyos in shul who does not remember how much he paid for 'Shelishi'. He has to say: it may have been one hundred, two hundred, or three hundred, but it certainly was not more than one thousand eight hundred - and therefore he brings one thousand eight hundred.
To the discussion we left off: one who obligated himself in a bar of gold:
What is the law regarding one who says that his intention was to bring gold as raw material, in the form of a bar, to the Beis HaMikdash? On this question there is a disagreement:
One approach: He brings the smallest quantity of gold needed to make the smallest gold vessel, that is, an amount of gold that is functionally usable in the Beis HaMikdash. There were tiny gold forks there, almost like those miniature forks used at a cocktail party, which are stuck into a slice of watermelon or a piece of cheese - a tiny piece of gold the size of a toothpick.
The approach of the Rambam: One who obligates himself in gold as currency, like paper money, intends money. Therefore we return to the previous rule: he brings an amount of gold up to the point where he can say "I certainly did not intend more than this", exactly as if he had obligated himself in money.