Negatives and the expression "faire exprès (de)"

Here is an example:

Tu fais exprès de m'ignorer? -- Non, je ne fais pas exprès de t'ignorer.
Are you intentionally ignoring me? -- No, I am not intentionally ignoring you.

In one of the quizzes a question was to translate the phrase "He didn't give the money back on purpose". The correct answer was: "Il a fait exprès de ne pas rendre la monnaie."

My initial guess would have been: "Il n'a pas fait exprès de rendre la monnaie." However, I'm now wondering whether this second sentence translates more like "he did not on purpose give the money back" (i.e. he did give me the money, but not intentionally).

Any ideas or clarification on this?

I have the same question as James, why isn't it "Il n'a pas fait exprès de rendre la monnaie."?

Il n'a pas fait esprès de rendre la monnaie. -- He did not on purpose give back the money. In this case he gave back the money, but somehow he did it not on purpose. What is being negated is the fact that he did it on purpose.

Il a fait exprès de ne pas rendre la monnaie. -- He purposfully did not give back the money. Here, he did something on purpose (il a fait exprès). And what he did on purpose is not to give back the money (ne pas rendre la monnaie).