I do have a 2001 Xterra . The airbag warning light is on. The Nissan Dealer says I have to change a sensor of the driver's seat belt ??? does that make any sense? the repair is quite expensive.

The seatbelt has a mini almost airbag built into the retractor. In case of an accident, it will pull it tight to help reduce bodily injury.

Thanks for your answer. Although, I wanted to know if there is a possibility to switch it off

No, once there is a fault, it's stored until repaired. It's done this way to let you know that it most likely will not work correctly in case of an accident.

Clear the code with an obd ll sensor