How can I apply the Trade-in value of an old phone after the purchase of a new phone and have the trade-in value apply to the new phone's cost?

I went to an Apple store with my old phone to trade it in for a new one. The salesman said that removing my old data and transferring it to a new phone would take a long time and I should buy the new phone from him and later transfer the data myself and then trade the old phone for a credit. However, when I attempt to trade in the old phone I only see the option of a gift card and not a way to receive money back from Apple against the new phone's price (as a credit). Is there a way to do this that I am missing? Did the salesman lead me astray?