Is what was eaten from the mistakenly served meal considered unlawful or lawful after the restaurant was compensated improperly?

What you ate is permissible, Allah willing, because it was purchased with a valid contract and was inherently permissible. Your compensation for your previous right falls under the issues of "al-Dhafr" (seizing one's right). It would have been better to use allusions or dissimulation instead of lying, unless lying was the only way to obtain the right.