Really? I can think of lots of things. If someone took your money from your wallet, that would be theft. If they refused to give it to you in the first place, that's not theft. It's pretty simple?
Maybe they took the money as a loan, intending to pay it back one day?
Sure, and they can provide evidence of that to a jury and the jury can decide if they're convinced of that. Just like they would in any other crime. I don't see the problem.