Sounds like the intent was a flat rate ad, which reverses to rev share should rev share exceed the flat rate.
Not sure if you had it worded right, so not sure if you would have a leg to stand on legally.
Ethically though, the program is in the wrong if that is the deal you made.
Media buys can be tricky.
Some programs I would not touch with a ten foot pole as far as media buys go.