I did some work on a project for someone else. When the project was complete the employer was late in sending me my paycheck. After a few months he wired the payment directly to my account and within the deposit he included extra money because he felt bad for the delay. Is this Ribbis? Do I have to return the money? Please provide sources.
There is a possibility that this is considered a rabbinic form of ribbis, though many are lenient for a one-time lump payment.
Certainly, there is no obligation to pay back the money.
The question of extra payment of this type is discussed by a number of authorities. Some write that it is permitted to pay extra for a late salary payment, and others are stringent (see Or Zarua, Bava Metzia 181; Beis Yosef Yoreh De’ah 160; Shach 176:8; Pischei Teshuvah 161:1 citing from Chavas Yair 190).
However, the authorities ruling stringently generally refer to an extra sum that is given on a time-delay basis (the longer the delay the greater the fine), so that this has a clear ribbis connotation. In your case, there is only a one-time lump payment, which is a single “fine” that is not time related. In this case, this will not be ribbis (see Yoreh De’ah 177:14).
Even if there is a ribbis concern, there is no obligation to pay it back because it is only ribbis me’ucheres, a rabbinic form of ribbis whereby there is no obligation to pay back the money.