Looking at the current price, it was not necessary to go to dark net ;-)

Anyway - do you know the person you gave your money? Your (maybe former) friend? How you gave him money - bank transfer? Cash?
Because if he said that he already made the transaction, he had to pay somehow.

Do you have any proof the transaction was made? If he has coins under any address, this information is public, he should be able to show it.

Or the problem is that coins are on his account in exchange and he cannot withdraw them? May he clarify what is the problem?