IF you son had set the fire, you would clearly be liable for the tree's cost. If you refuse to voluntarily pay, your landlord could sue you for the money, or take it out of your security deposit. If they sue you, they would have to win in court, based on witness testimony (e.g. the people who claim they saw your son set the fire) that your son caused the fire; you will have your chance to put your testimony and evidence (and that of your son) out, too, to refute the landlord's case, and the court will decide who is more likely right. Similarly, if they withhold money from your security deposit and you feel it was wrong for them to do so, you could sue the landlord for the return of the money, and a court will again hear testimony and decide what most likely occured.