Depends on the country your in and their laws. Buying sex for someone else is not a illegal here. You're not selling it, buying it for yourself or pimping off the girl. It might come under "Sex Tourism" laws.
Check with a lawyer first. I could and did offer it as a prize. Did it twice, one winner brought his girlfriend, the other was hitting unsuccessfully on Eva.
But he did get laid a few times while here.