How about this:

I'll promise to invest $500 in BTC.
You put $687.50 in BTC into escrow. (Super Secret Money guy pays fees). Instruct escrow agent to fwd those funds to me after 10 days. (187.50 is 37.5% profit - based on an average of your range for $500 investment and a 10 day unit).
Once i have confirmation of the escrow deposit and a signed email from the escrow agent that they will fwd my funds in 10 days, I'll send you $500 worth of BTC.

k?  how's that work out for ya?