Help to remove a patch

I was going to sell my late 2015 intel iMac but the buyer installed a firmware hack to upgrade the is from Monterey to Catalina. The buyer dropped their price so I took the Mac back.

I have the Mac back in my possession and want to remove what they have done

Booting internet recovery will erase the Fusion Drive but I need to revert the base system. (macOS Fails to install)