Hey, magenta1327106! Forgive me, but I'm going to restate this first and make sure I understand what's happening here.
Do you have an iPhone on a 24 month equipment installment plan and pay monthly for JUMP! insurance? If that's you're situation, and your iPhone is almost paid off (with $200 left I'd guess you've only got five installments or so), then I'm guessing you called in to complete a JUMP! upgrade. When you JUMP!, you begin a new EIP for the new phone (in this case, you'd pay a down payment for the iPhone X and the rest would be split up over 24 months), and your existing EIP (the financing agreement for your current iPhone) goes into something called a "JUMP! Return Pending" status.
You don't have to return your existing iPhone, but the remaining installments will continue to bill out monthly along with the installments for your new iPhone X, and until that EIP is complete you wouldn't be able to use your JUMP! benefit on the iPhone X (if for some reason you wanted to switch phones again before the current iPhone is paid off). If you do turn it in, the remaining balance that you owe on it (the $200 you mentioned) will be wiped off of the agreement, and you'll only make the payments for your new phone.
From what you're saying, it sounds like your intention was to pay off the balance on your original EIP, then trade the phone in to participate in the current $300 off iPhone offer (for which JUMP! orders aren't eligible). Systematically, this actually shouldn't be possible, because a trade-in has to be created when the order is created, and an IMEI with a device financing agreement balance should be rejected by the trade-in system. To pay off your device and then participate in the offer, you'd need to initiate the pay-off first, before the order was placed.