"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 emphasis is placed differently with different particles,
The phrase "There is money in the wallet" presents "money" as new information and answers the question of "What is in the wallet?", so the recommended answer marks "money" with the new information particle が
Your answer marks money as old contextual information with は, implying that it is already known by the listener that the money exists. This would translate better to "The money is inside the wallet" in response to "Where is the money?"
の中 also isn't really necessary since it can be understood from context the relation of the money to the wallet, though including it should be fine and just may not be included in the answer list yet.