It's a weird system. From 1 April 2017, to register the car, the dealer will have already paid the first year showroom tax. But because the car is changing keeper, you will have to re-tax it at the rate of £140 a year. The dealer can claim back a proportion of that £140 or a proportion of the showroom tax depending on which is lower. This assumes the car is not a hybrid (£10pa less) and is not over £40,000 (£310pa more).