Car valuation when purchased for exchange

Hi,

We have  acr scheme in our company where employees are allowed to buy cars. We have acse where an employee is exchaging  acr for a new one. How to value the car being purchased? THe issue being that the original car is not in our company books. It is right to apply AS 10 and value at NBV of car exchanged plus paoyout by our company? Please let me know correct valuation method.