Baba Metzia 4:7

A difference of four silver maos per sela is considered overcharging. For a shortage of two silver maos to the sela, the court can obligate an oath. Admitting to a claim is for a minimum of a prutah. The Sages established a prutah as the minimum value in five cases: admitting to a claim; betrothing a woman; benefitting from Temple property in order to violate the law of misappropriation; needing to announce a found object; one who steals and swears falsely needing to travel to return it, even as far as Media.

Baba Metzia 4:8

There are five cases in which a fifth is added to a principal: if one eats trumah, trumas maaser, trumas maaser from doubtfully-tithed produce, challah or first fruits; if one redeems produce of a fourth-year tree or second tithe; if one redeems property that he sanctified; if one benefits from Temple property to the minimum value of a prutah; if one steals the minimum value of a prutah and swears falsely about it. In all of these cases, one must add one-fifth.