McHenryB :
1. You should take it back to the repair shop and insist that they reinstall the proper OS.
2. Depending upon the age of the Mac you can use Internet install to reinstall the OS. You don't need to buy the OS again as Apple will have a record of your authorization to use the OS.
3. But the most important is 1. You are saying that they have defrauded you, so you must insist that they reinstate your computer as it was. Otherwise go to your local trading standards office or take them to court. (In the UK this is very easy using the Small Claims Court.) You shouldn't let them get away with this illegal behaviour.
Idk how legitimate that claim is given it was apparently a HDD repair, and most likely the data with the OS on it could have been corrupted or lost. They're in line there to place a temporary working operating system on it, no matter what quality, just so long as its in working order.
We do the same thing with Linux if people have effed up Macs and the OS is corrupted, and sell a new copy of Mac OS to the consumer, since it shouldn't come at our cost.