I think that decision was made by Harper/Boras. Remember that he was announced as an OF at the draft.
But I'm fairly sure that I remember that it was the Nats who made that decision and announced it prior to the draft (since it was know that we were going to draft him). Boras has no authority to make any such decision.
Although I suppose it could be argued that Harper does. Even then, I'm not sure. Does a player have the authority to determine what position he plays? Look what happened to Soriano.