The power grid has become a critical infrastructure, which modern society cannot do without. It has always been a challenge to keep power supply and demand in balance; the more so with the recent rise of intermittent renewable energy sources. Demand response (DR) schemes are one of the counter measures, traditionally employed with large industrial plants. This paper suggests to consider data centers (DCs) as candidates for DR as they are large energy consumers and as they are able to adapt their power profile sufficiently well. To unlock this potential, we suggest a system of contracts that regulate collaboration and economic incentives between the DC and its energy supplier (GreenSDA) as well as between the DC and its customers (GreenSLA). Several presented use cases serve to validate the suitability of DCs for DR schemes.