I suspect the ad buys are already contracted and in the books. Unless there’s proviso that Trump is in the mix, they’ll go forward.
Fox News may keep the money, but lose a lot of goodwill in the process.
That said, I wonder if the contracts don't have a clause to cover contingencies like this.
The earlier contracts stipulated that Trump had to be present.
He said advertisers didn't even contract directly with the networks anyway. He worked for a third-party broker that would buy up large blocks of advertising spots at discounted prices (due to the volume of business), then sell them off individually to advertisers.
The only incentive Fox has at this point is to protect its image, not its revenues. If their presidential debate has lower ratings than a broadcast of a campaign event for one of the candidates, then it would be a major embarrassment for them.