Shevuot (Oaths) 44 - Sages-Decreed Oaths
All who swear an oath decreed by the Torah, swear to free themselves from liability. However, the Sages have decreed additional oaths, and as a result of taking these oaths, one swears and collects the money due to him. They include: a hired worker, a victim of theft, a victim of assault, one whose opposing side is suspect in regards to oaths, and a storekeeper regarding that which is written in his ledger.
What is the case of a hired worker? If a hired worker tells his employer, "Pay me my wages!", and the employer says, "I already paid!", but the worker says, "I never received them" - the worker swears that he has not been paid and then collects the wage. The reason for this oath is that the employer has many workers and may think that he already paid that one.
Art: Thomas Pollock Anschutz - Iron Workers at Noontime