Why don't you ask them?
Usually Best Buy gives you a gift card with the value of the trade.
You will get $700 refunded back to your original form of payment because that's what you paid, and the remaining $700 will be issued as best buy gift card in your example.So for example. Phone is 1400, trade in is 700. You paid 700 out of pocket.
Return value would be 700. You won't get more than your out of pocket expense.