I actually like the fact you can only challange them once a day, and the fact you dont get any exp once you've lost.
however I don't like the fact you won't get ANY money.
like there has been said above theres quite a few uses for your money in this game and theres other ways of earning money faster (and more) than doing the E4 once (each day). So I feel like it wouldn't be unbalanced to just make them give a (small) amound of money