Where'd you hear that one? I'm not aware of any recent story matching that description. Belgian construction sites do turn up old stuff now and then - coins, artifacts, whatever - but a €9 million find by a student would've been major news and I'm not finding anything solid on it.
That said, cities absolutely do sit on buried treasure in the literal sense. Archaeological digs regularly uncover Roman coins, medieval jewelry, lost valuables from centuries back. Most of it gets reported to authorities and ends up in museums or gets split between the finder and the landowner, depending on local law. The odds of someone casually digging a sewer and hitting nine figures worth of anything are basically zero, though - even rare finds typically run to thousands or maybe low hundreds of thousands at most.
If you've got a link to the story, worth checking the source. A lot of these viral posts get embellished or straight-up fabricated as they spread. Real construction finds do happen, just usually way less dramatic than the headlines suggest.