If you want a guarantee, I'd recommend only paying once the guarantee is met. It's much easier than trying to get people to return money they've already assumed was theirs.
I've done tech recruiting for many startups and I typically charge 0 upfront, then 10% of base salary on the employee's start date and 10% after 6mo of full-time employment. In Hurry's case, maybe pay $7.5k on date of hire and $7.5k after 12 months.
I have plenty of folks in my network that would be a great fit and the $15k incentive (for my half) is respectable, but I wouldn't want to work under the terms as proposed unfortunately. A small tweak to the payment structure and this would be an excellent program that I'd jump at.