Failure to do a service according to the book

I bought a 2nd hand car from a dealership in June 2015,it's a 2007 C200k w204,it had 84000kms when I bought it.I bought it under the impression that it had a full service history as the previous owner did his services at Mercedes,most were done at Bryanston and one at Sandton. In July 2015 I started experiencing a problem with the automatic gearbox, it was taking time to change gears,so I sent it to Mercedes M2 city on 6 July 2015 to have a look at it and they said the vehicles gearbox was supposed to have been serviced at 50000km but it wasn't done so I had to do it at a cost of R5333.80. I contacted the dealership where I purchased the car to say what had happened and they said that I must contact Mercedes which I did last week Friday.I phoned the Bryanston dealership and spoke the the service manager Andrew who said the 50000km service was done in Sandton and the 60000km service by them but the gearbox was never serviced,according to him when a car comes in for a service they do what needs to be done according to a schedule so they don't check the gearbox but according to the service book it needs to be serviced at 50000km or 3years. The car was under a maintenance plan at then.