Ask HN: Handling Posting Fees/Reward Money

I’m working on Contest site where people can post a contest for $5/contest. for prize money of 15-25$. Earlier I was thinking of taking the (fee + reward) money upfront from the poster so I can reliably give prize to the winners. But now I'm thinking of just charging the posting fee and let poster handle the transfer of reward to the winner on contest completion. This would avoid the middle man fees and winners will get more without any cuts.

What are your thoughts on either approach?