May sound harsh but a BMW X5 40d must be £25k worth of car. Would anyone on here be happy if their £25k investment was leaking oil after a couple of months?? My advise would be get it sorted and send the customer away happy. I'm guessing maybe it's an 10 hour job, if you have a workshop it's a few days work if not I'm guess you will be paying a trade rate. No one likes a £600 bill on a car that is sold but if it gets to court expect a far larger bill then that. Keep the customer onside and he may chip in a bit.
We all sell cars in the attempt to make money but we do have a legal obligation to our customers too, it's what sets us aside from the cowboys who give the trade a bad name.