https://www.theverge.com/2018/11/7/18073488/oneplus-6t-tmobile-300-trade-in-offer-deadline

Here’s just a short PSA for people interested in buying the OnePlus 6T on T-Mobile: the carrier’s $300 trade-in promotion is ending tomorrow. There will no doubt be other trade offers down the line, but the one announced back at the phone’s keynote was particularly generous; T-Mobile will give you $300 for any previous OnePlus handset to put towards the 6T. There’s also a long list of other phones that are eligible for the $300 discount.

The $300 is applied over 24 months when you buy the OnePlus 6 with a monthly payment plan; you can’t get the $300 and then just pay the rest immediately to buy the 6T outright. The credits will begin two billing cycles after you take home the 6T.