I'm wondering why Lucius tried to get out of the deal.

I see it as a status game. Previously Lucius had suggested that Dumbledore take Hermione's place in Azkaban, not because Lucius thought there was much of a chance that Dumbledore accept this offer, but rather to strike at the idea of Dumbledore's supposed heroic altruism.

Now when Lucius named 100,000 galleons as the price to erase the debt, he was trying a similar tactic -- Lucius didn't expect that Harry accept this, Lucius wanted to strike at the idea of his enemies being heroic and altruistic.

But Harry accepts - which in terms of impressiveness is a blow in Harry's favor and against Lucius, despite the fact that Harry's now endebted to Lucius, and because the sum named is so outrageously large.

But then Lucius thought he saw a opportunity to strike back at Dumbledore and Harry, because he thought they were pretending a play to raise Harry's status even further (not just heroically altruistic, but super-heroically tough, able to destroy Azkaban, all by himself).

So he attempted to call Harry and Dumbledore's bluff. And it failed again.