Here's my recommended approach:
1.
Advertise a vacancy for a programmer. State requirements (languages, experience, background, etc.)
2. Filter and interview applicants that meet your criteria.
3. Short-list and offer job to successful candidate(s)
4. Enter into an employment and confidentiality contract with successful candidate
5. At this point you can put them on the confidential project and pay them the money.
This is a true and tried approach that has worked for many organisations.
The approach that you're attempting is almost guaranteed to cause a failure because it has so many weaknesses.