conscience money

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Money paid in compensation or atonement, as for a dishonest or morally objectionable act.

GNU Webster's 1913

1. n. stolen or wrongfully acquired money that is voluntarily restored to the rightful possessor. Such money paid into the United States treasury by unknown debtors is called the Conscience fund.

WordNet 3.0

1. n. payment made voluntarily to reduce guilt over dishonest dealings