Come on now- in this case the dealer is correct. You pull in with a scuffed bag and say another dealer did it, but you'd like them to fix it? If a particular dealer caused the damage, you need to take it to them and have it corrected, not ask some other dealer to eat the expense, especially since this sounds like it's not a warranty issue, but a case of damage incurred while getting service done, type of thing. The second dealer has no way of knowing who caused the damage- you or the other dealer, so you will need to hash it out with the dealer who you say caused it.