Ketubot, Chapter Six, Mishnah Three
This mishnah discusses the sums of the dowry written in the ketubah. Customarily the woman would bring a dowry consisting of money, objects and potentially land (our mishnah does not discuss land, because its location and not value was written in the ketubah). The amount of dowry that the husband would write was the amount he would be obligated to return to her upon divorce or death. If she brought a dowry of money, he would write a higher sum than she actually brought since he benefits from the use of the money. If she brought movable property into the marriage, he writes an amount one-fifth less, for it was customary to overestimate the value of the dowry to make the bride and her family sound richer than they really were.
1) If a woman agreed to bring her husband one thousand denarii he must agree to give her a corresponding sum of fifteen maneh.
2) As a corresponding sum for appraised goods, he agrees to give one-fifth less.
3) [If a husband is requested to enter in his wife’s ketubah] goods assessed at one maneh, and these are in fact worth a maneh, he only [must agree to] a maneh.
i) [Otherwise, if he is requested to enter in the ketubah:] goods assessed at a maneh, his wife must give him thirty-one sela and a denar, and if at four hundred, she must give [him goods valued at] five hundred.
ii) Whatever a bridegroom agrees to give [his wife in her ketubah] he writes one fifth less [than the appraised value].
Section one: If a wife brings into the marriage 1000 denarii of cash, the husband writes that he has received 1500. This is because for the duration of the marriage he benefits from the use of the money.
Section two: If she brings in goods whose value has been assessed, the husband need write in the ketubah only a fifth less of the value. The primary reason, according to most commentators, is that dowries tend to be overestimated (just like people planning weddings tend to be overcharged!). The husband should not have to pay for his overestimation. However, the mishnah notes that if he wrote 100 zuz in ketubah and she brings in goods that are actually worth 100 zuz, he cannot ask for more.
The mishnah now illustrates two cases where the amount written in the ketubah is less than she actually brings into the marriage. If he writes 100, she must bring in 125 denarii (sela=4 denar). That is to say, he has reduced the amount by 1/5. Similarly if he writes 400 zuz worth of goods, she must bring 500.
Section three: The husband too, when he promises to bring a certain amount of goods into the marriage (for instance clothes or perfume for his wife) writes the value at one/fifth less. This is true because these goods are also generally overestimated.