Hi Clement,

Assuming it has the popular 2.0-litre TDI diesel engine, the belt must be changed every 210,000km, regardless of time. With 140,000 miles on the clock, it should have been changed already. If it has not been, I'd worry about what other maintenance has been skipped or not done on time, such as oil changes, etc. 

Regardless, while it may be overdue, there's no way you can force the seller to do the work before you take delivery. You may be able to use it as a way to negotiate a little off the price, but I'd suggest looking elsewhere for a car with a complete service history.