Why would he even bother to pay someone if it's online? He could easily search up the answers himself, or you know, just use the notes. There's little to no reason to fail an out of class exam...
That being said, if you're truly worried about that situation, report it, don't mention a name though. Just say you have concerns about this situation, and that you heard people already trying to bribe the top students to do the test for them. Hell, even the professor alone shouldn't want to hear that and try to change the situation.