Many payment companies give away free POS software if you process payments through them.
Older versions (like v8 or v8.1) sometimes offered one-time payments with varying years of free updates. Version Upgrades: descargar soft restaurant free full link
Open-source means free to download, install, and modify. But you need your own server (or cloud hosting) and technical skills to set it up. No monthly fees, but you pay for installation/support. Many payment companies give away free POS software