If you can leave that aside and just charge the freelancer with that 3%, let's say. If this doesn't make sense, you can always set a minimum charge, like 5 euros or 3% of the transaction, which ever is bigger.
That may be ok, but is definitely far less sustainable, as you have to keep creating new matches (as opposed to being paid for an existing match continuing).
However, the most freelancers get short jobs and many clients, as in the original article is specified, so this just simply works.
EDIT: it seems that as currently implemented, even Upwork is bumping in a search the ones with many hours/money earned. So cheating would be a bad decision, since it may drastically affect your visibility on the site
And may be one of the primary reasons for this "feature". I don't blame them, they are a marketplace and should encourage transactions on their platform.