When you part exchange, you trade in your current car for cash which is then put towards the deposit on your next car. In a part exchange, you and the retailer must first agree on the trade-in value of your old car — how much it’s worth will vary based on its condition, mileage and other factors. Once the trade-in value has been agreed, the retailer deducts that amount from the price of the new (or used) car that they’re selling you. You’ll then either pay off the remaining balance in cash, or take out a loan or finance agreement to cover it.