You could probably sell your iPhone 6+ and get enough to pay off your ETF, buy a used iPhone 4 to trade in to T-Mobile for the deal, and come out with some cash left over.
EDIT: Though it says not eligible for device pay off. It doesn't say it's not eligible for covering ETF. They list this as two separate benefits on their site so they might offer it.
I got a tweet back from T-Mobile after replying to John's tweet lol. Here is what they said in case anyone else is curious:
"It can be combined with the ETF reimbursement offer but not the Carrier Freedom offer that pays of your device balance."