I bought an X5 that had something similar when I checked the history but it was only one clerical error and the log book was stamped and mileage noted each time.
I would have a look at the log book and check that, I assume it will only have the original inspection done and will be due a service quite soon? Dealer should also be able to verify the mileage from BMW records if the car has been in for warranty work? I would want something in writing from dealer and haggle price as well as it will inevitably make the car harder to sell on in the future unless the record can be cleaned.