Letter fromGerman Car Hire company stating there is minor damage to car, nearly 600 Euros to pay

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Dh rented a car in germany for 24 hours. He says there were no damage to the car when he returned it. The hire company did not come out to check the car with him. Now we have a letter saying there was minor damage, to the value of nearly 600 euros, and unless we pay there will be additional cost and court action.

I think we have basically just been stung.

Any advice as to what we could do in this situation?

I think if they signed it in again as being ok, then they can't charge, as they can't prove it was done while you had it - could have been one of their staff moving the car/someone bumping it later.
And in any case, you can DEFINITELY argue about whether it'll be 600E, bargaining works...