As others have said, try and go through the dealer first - since this is in the back by the motor I don't know how anyone could claim it was a rock or stump without a long scratch going forward, or from the rear without damage to the lower unit or prop. Very high probability this was done unloading or pre-rigging. But we know how people tend to act, deny deny deny because at the end of the day someone is going to have to stroke a check.
Stick to your guns and hopefully the dealer will want your business for many years to come and try to resolve this amicably. On the flip side don't burn your bridges with the dealer either because there will be times you need them for parts, service or a bigger warranty claim and will need a friend. Hope you get it resolved quickly and to your satisfaction!