If I remember my Business Law from College and by no means am I a lawyer, contracts have to be specific. There has to be a "meeting of the minds" "the Agreement" & "Consideration". Since the keys were just handed to you there was no "meeting of the minds" you did not sign anything so there is no "Agreement" and you did not agree to pay for anything and did not pay for anything, so there was no "consideration" given. I believe like others here have stated, the dealer who is the customer at this point and the rental rep dropped the ball and are trying to bag you for it. You cannot / should not be held responsible for something that you never acknowledged or were asked to be responsible for.