Menahot, Chapter Thirteen, Mishnah Four
Todays mishnah deals with a person who volunteers to bring an unspecified amount of money as a donation to the Temple.
1) I take upon myself to offer gold, he must bring not less than a golden denar.
2) Silver, he must bring not less than a silver denar.
3) Copper, he must bring not less than [the value of] a silver maah.
4) [If he said] I specified [how much I would bring] but I do not know what I specified, he must bring until he says, I certainly did not intend to give so much!
Section one: Since the smallest gold coin is a golden denar, the person who pledges to bring gold must bring at least a golden denar.
Section two: The same rule applies when it comes to silver.
Section three: If someone pledges to bring copper, he must bring copper that is equivalent in value to one silver maah. A maah is 1/24 of a sela, and a sela is equivalent to 4 denars, so a maah is 1/6 of a denar.
Section four: In this case, the person doesnt remember how much he pledged. He must therefore bring the maximum amount he might have possibly pledged. In other words, we might say to him, Might you have pledged 10 denars? If he says yes, then we would say, Might you have pledged 100 denars? If he says yes, then the number keeps going up until he hits a number that he knows he definitely didnt pledge.
We should note that in all of these cases we are dealing with a person who honestly wants to fulfill his vow. There is no concern that the person might lie and say I could not have possibly pledged to bring such a large amount when he really might have, because if we were concerned about him lying, he could have just lied and said that he remembered having pledged a lower amount. Rather, in all of these mishnayot we are dealing with a person who honestly wants to figure out how much money he owes to the Temple.