You are absolutely not liable in this situation. You informed the complex (inform them one more time in writing) and contact your state's consumer protection office that handles landlord tenant matters (usually the city attorney or state attorney general), explain the dog is off leash and over 30 lbs and it is the duty of the landlord at this point to make sure other tenants do not breach laws (off leash) and policies (over 30 lbs) because it is a health and safety issue for other tenants. If this does not resolve the matter to get your security back, sue the landlord first.