Translation:There is money in the wallet.
The は makes the wallet into the topic of the sentence. Something like "Concerning the inside of the wallet". You usually do this to stress one piece of information in the sentence over another. This is the topic, and now I'm going to say something important about this topic. Kinda like "Inside my wallet? There's MONEY."
Yes, this could be "I have money in my wallet" as well as a more general "there is money in the wallet".
But, as you said, the money is the actual subject here. In English, "in my wallet, there is money", feels like you're stressing the fact that the wallet yours rather than someone else's. And that -in Japanese- would require you to specify ownership. I.e. 私のさいふ...