"There is money in the wallet."
I can't quite put my finger on it, but it sounds really odd, like the sentence lacks a subject, or as if it's a general statement, but in the wrong order. E.g. "in wallets, money is there".
In Duo's sentence 財布(さいふ)には means "in the wallet", with は indicating that this is the topic/setting, and が makes お金 the leading subject. "There is money in the wallet".
Sorry if that's of little actual help.
The verbs aru and iru go with ga, not wa. When indicating a subject , I have never seen wa used with the aforementioned verbs, only ga. That is at least how they taught my in my first year of university where I am studying Japanese.
Someone should teach that to Duo. Another sentence in this lesson does not accept が, only は for ある. I flagged it though.
It feels to me that that would be saying "The money in the wallet exists/is there", presupposing that it is in the wallet instead of stating it.
As I mentioned above in a previous comment, the verbs aru and iru take the particle ga and never wa. That is at least how I've been taught during my studies of the Japanese languages so far.