You both sign a contract that explains that they write a good enough
article by a certain date, then they get the $100 and you get sole
copyright to it, and then you need not pay them anything further. It's
a standard work-for-hire sort of contract that you can probably find
examples of in legal books about that sort of thing.
Mark
You may find your pro-writer is no longer interested.
Standard industry best practices for this type of work product is a one time
insertion/use subsiquent use requies payment of a royalty fee.
BTW 'work for hire' is the exception not the norm.
If you're unsure you can add on the stipulation that you own all
rights to the article they produce.
You'd be surprised how many writers are just trying to make a buck and
don't care to give out rights to an article. They aren't really out
to make a name for themselves, unless it involves being copywriter of
the year.