Pledges to buy back tokens issued by platforms in decentralized finance are becoming increasingly common. We develop a tractable model for the exchange rates of platform tokens that incorporates user demand, investment demand, and buyback pledges. We derive closed-form solutions for the valuation of tokens and the time required to fulfill the pledge. Buyback pledges can increase the value of the tokens, but manipulation of the token supply by market participants may make the pledge more expensive than intended. Sufficiently aggressive buyback pledges induce purely financially motivated investment in platform tokens, which could trigger regulatory classification as securities.