My advice is that you need a lawyer to review what you are trying to accomplish. From what I understand about what you are saying, I think it is an unreasonable request of the Buyer and it would be highly unlikely for your neighbor to go along with this request. If they do, I have a bridge in Brooklyn that I would like to sell them. All kidding aside, the fact that the Buyer is requesting this, leads me to believe there is more to the story here. In any event, generally speaking, anyone who negligently causes harm to another's property would be liable without a signed document. I also understand when you say garage that are describing a residential a garage where your neighbor parks their cars, not a full-blown gas station.