OpenBravo POS used to be ok, it even worked with barcode readers. But you could also manually punch up a sales receipt and it would adjust on hand numbers automatically. But Open Bravo has moved on to a cloud paid model. However there a few forks from the before time, like Unicenta, but I've never used that one and can only vouch for what it was back in the day. Source: almost 3 years ago

I would recommend going with https://opensourcepos.org/ or https://unicenta.com/. Source: almost 3 years ago

Unicenta works pretty well on a POS that I salvaged from a shop closure last year (no reason for doing so - just thought it would be cool to have one for the hell of it). Runs on Linux as well as Windows if you want to escape Windows Embedded 7. Source: almost 3 years ago

Looks like Unicenta is an alternative POS that can be self-hosted and is licensed as GPL-3.0-or-later. Source: about 3 years ago

He needs some kind person that would take the time to explain him how to that kind of "migration", also explaining him what is the difference between doing this and a low level copy with Clonezilla or PING. Source: about 2 years ago