From Longman Business Dictionary
part-exchangeˌpart-exˈchange noun [countableC, uncountableU] British EnglishBrECOMMERCE a way of buying a new car, television etc in which you give the seller your old car etc as part of the payment, or a car etc given as part of the payment for something newSYN AmE trade-inHe sold the car in part-exchange for another vehicle.
—part-exchange verb [transitiveT]We have fully warranted used sports cars for sale that have been part-exchanged for other vehicles in our stock range.