The problem that you have is proving that the gardener or his/her assistant was careless in not locking the gates to the enclosed area properly so that the tenant's dogs got out resulting in them being caught by the SPCA where the fine for their releaseis $300.00.If you or the tenant can prove that the gardener or his/her assistant did not properly lock the gates and the dogs got out, the gardener would be responsible for the results of his or her conduct that caused the damages, namely the $300.00 fine.You as the landlord could be liable if you hired the gardener as your agent, the gardener was negligent and and the gardener cannot pay the $300.00 amount.