I have recently come across this expression while I was talking to my colleague on the phone. I cannot reveal the identity of my colleague, but let me tell you what happened to her the other day.
Apparently, she forgot to turn up on a given day of the week when she should be really giving her make-up class! On the following week, she walked into her English class, looking rather sheepish, with a box of sweets to be handed to each of her student as a token of saying ‘I’m sorry’.
Now, when she was describing the above situation on the phone, I just interrputed by saying, ‘So the sweets are kind of bribe to the students, aren’t they?’. To this she said something like: ‘No, they are not a bribe to the students, but they had certainly an effect of greasing the wheels.’