Kesubos 6:3-4

Kesubos 6:3

If the bride agrees to bring a dowry of 1,000 dinar into the marriage, the husband writes a kesubah of 1,500 against it. If she brings a dowry of movable property that is appraised, he writes a kesubah worth that amount minus 20% (because appraisals were typically inflated). If the property was appraised at 100 dinar and was actually worth 100 dinar, he writes the kesubah for 100 dinar. If she wants him to write a kesubah for 100 dinar, she must bring property appraised at 125 dinar (to account for the 20% reduction that will be made); if she wants a kesubah of 400, she must bring property appraised at 500 dinar. The things that the groom agrees to provide in the kesubah he provides minus 20%.

Kesubos 6:4

If the bride agreed to provide cash, a silver sela (i.e., four dinar) becomes six dinar in the kesubah. For her chest of beauty products, he must provide ten dinar for every 100 dinar of its value. Rabban Shimon ben Gamliel says that all adjustments in kesubah negotiations follow the accepted local practices.