T-Mobile fired up a buy one, get one (BOGO) deal on the Galaxy S8 shortly after it launched. While there were caveats, it was a good value. Now, that deal is back, and the upcoming Moto Z2 Force getting the same BOGO offer. You probably want the GS8, though.
Not all customers will be able to get this BOGO deal, so check the terms. The gist is that you must be adding at least one line of service, and you have to purchase the phones with a payment plan. You buy both phones and make your monthly payments for a while. If you cancel service, the full balance is due. Then, some weeks later a pre-paid MasterCard will arrive in the mail with the value of one phone on it.
For the Z2 Force or Galaxy S8, you will get back $750. The Galaxy S8 Plus is also included, and that one will get you the full $800 back. The money can be used to pay off your phone immediately, or you can blow it on something else. Note, we're still working on the Moto Z2 Force review, but I'm not feeling super-confident about that device right now. Pre-ordering it seems like a bad idea.