Mod, I hear your point and I concede it is not a simple or straightforward issue here. However, since the rules do not describe any halachic workarounds, the contract is that if you win they are required to pay you 100x per year for 10 years. But you can exempt them from the full obligation by accepting half as an immediate payment, which is an issur ribbis drabbonon. Stated another way, if you want your money now you only get half. They can fix this by saying they will pay 1000x whether you ask for it upfront or over time.
From the other perspective (since it is ambiguous) maybe the prize is 500x right now but if you let them pay over time you get 1000x. Then it is ribbis doraisa. The winner then might be able to prevent the ribbis by refusing the alternative offered and allowing them to pay 500x over 10 years or immediately. But what kind of business is that for a kiruv org to leave control of the ribbis issue up to whoever wins? Chances are the winner will have no idea what ribbis even means.