Somebody makes a dedication to the treasury of the Beit Hamikdash (Holy Temple), and he dedicates a gold coin, but he doesn't specify how large a coin he's talking about. He's not allowed to bring less than a dinar. It's a coin in this location that was the smallest possible currency, and therefore it has to be at least that. But if he just declares that he wants to bring gold, it doesn't say that he wants to bring a coin. We make him donate a piece of gold that's large enough to make a golden fork. A fork was the smallest golden utensil used in the Beit Hamikdash.
Kesef, lo yifchot midinar kesef. If he has a swear to donate silver—a silver coin to the Beit Hamikdash—he has to bring a minimum of a silver dinar. He has to bring the silver dinar, which is again the smallest silver coin in currency at that point in time. There were other smaller coins at different places and different times, but we're talking about a location where a dinar was the smallest silver currency used. Nechoshet, lo yifchot mime'ah (coin denomination) kesef. If he made a swear to bring copper, he's not allowed to bring less copper than an amount equal to the cost of a silver ma'ah coin. He doesn't have to actually bring a ma'ah coin, which is silver, but he has to bring copper. He has to bring the amount of copper equal to a silver.
If a person says, "I know I made a donation, either gold, silver, or copper, and I did specify an amount, but I don't remember what amount I specified," we say, hu mevi ad sheyomar. He has to bring a high enough amount where he says, right to that point—the limit where he says, "It couldn't have been that I donated that much." He has to bring enough of that type of metal to the point where he says, "I guarantee you, I never would have made that type of donation in my life." So he has to get right under that threshold, which is basically the limit, according to his wealth, of what he ever would have donated. That large an amount would be sufficient because it definitely covers what he donated.