Can journalists have their stories "stolen" by pitched editors?

I am pitching a story to a number of large publications about a boxer, whom has knowledge that I am pitching his story and he is all about it.

What if an editor likes the pitch, but decides to have someone else write it (perhaps he thinks I’m too green or would rather assign the story to a person with whom he/she has a working relationship with)?

Is it possible to ask my subject to keep this story exclusive to me…?

Still new to this game, but learning quickly.

Thanks, Flutherites. :)