I thought there was already a thread on this, but I can't find it.

The bribe seems hidden, but I've got a theory now that I'm rereading.

Kreacher comes back with Mundungus and Mundungus tells the story about the locket -- A certain toad-faced ministry official comes up and threatens him. Threatens to fine him, but she takes the locket in return for not fining him. So Mundungus giving the locket to Umbridge in return for not fining him is the bribe.

While I think that's a chincy bribe, it would qualify. AND it is huge because it gets the locket into Umbridge's hands.