Var NFN: you don't deliver, they fail and don't pay
Var DFN: you do deliver, they fail and don't pay
Var DSN: you do deliver, they succeed and don't pay
Var DSP: you do deliver, they succeed and .. pay you? Why should they?
Your chance is 1/4 at best and 1/8, 12.5% realistically.
You credited them with $250k (1/40 of a potential revenue?), you won't find a quarter of million lying around.
As others already said - forget about that $250k. You now have a gambling chance to get something (way lower than $250k), but only if you demand the cash upfront.
PS: technically there is a variant NSP: you don't deliver, they succeed .. and pay you. It is only interesting in what it lowers your chance to 1/10. *grin*