If it IS a consulting arrangement, then:
1. Make sure you are allowed to engage on such terms with your current terms of employment. You could very quickly find yourself out of a job with your current employer.
2. Make sure you're compensated appropriately. I know that if I were to do some short term work on a consulting basis, I'd be remunerated well more than "my salary plus a small margin". More like a multiple of my salaried hourly rate. Your industry may be different, but this guy could just be looking for some high value consulting at a bargain basement price.
Finally, I wouldn't hop on a plane without knowing what they are expecting from you. If it's consulting, then they should be upfront. If they are looking at possible fit for a recruitment opportunity, again they should be upfront. If they aren't prepared to elaborate, then I'd run a mile because there are a range of things it could be that have the potential to leave you in a very difficult situation.