Get your old hard drive back. He's not entitled to keep it. There are all sorts of freebies to recover data, and I'm happy to step you through it.
Tell me a little about your machine: desktop/laptop? Operating system: win11 or earlier. Or Linux/Mac.
Type of hard drive: Disk drive (the older, bigger ones) or SDD - solid state? And anything else you care to pass on.
TBH, he sound like a bit of a dick. Did he give you a reason for replacing the drive? A virus or trojan doesn't usually need installation of a new drive. Did he say precisely what was wrong with it and list the problem he found?
If you can get your old drive back, I can also help you with partitioning your drive (depending on it's size) and moving your own stuff off onto the new partition. Never lose your files again. 
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