From Longman Business Dictionary
backhanderback·hand·er /ˈbækhændə-ər/ noun [countableC] British EnglishBrE informal an amount of money or something valuable that someone gives you to persuade you to help them or do something dishonest for themSYN BRIBEA former Ministry of Defence worker has been found guilty of accepting £1.5 million in backhanders from arms companies.