My landlord is now asking for $600 deposit for my dog a quarter way into the lease.?

The lease i signed said it was okay for 1 dog and never mentioned a need for a pet deposit, at least in this lease. Can he really demand a $600 deposit immediately for my dog to stay? He is asking for that because of a minor damage; scratches in drywall. But isn't that what the $725 deposit was for in the first place of moving in there? In case something damaging happens to the property??