The oral-promise part of this would seem to spell trouble, as it sets up a your-word-against-theirs situation. Could be the agent, if questioned, will just say they don't recall any of it, and there's probably not a tape. Under those circumstances I think you might get the free rental day, but it'll take an admission of error by the agent or a tape or something more substantial to get a money reimbursement, I'm guessing.