Why would this guy have any reason to betray them. He's strong enough to kill them both upfront. If the girl had a bounty on her head or someone wanted her dead why would he get the MC involved and drag him along.
Oh, you can still put him in a slave contract, just let him see his mom from time to time and don't be a smug cunt about tricking him into signing his life away.
100% the same plot as all the other "rebirth/rise/etc" of the "strongest/immortal/heavenly/sword god/etc" urban cultivator. Have any of these protagonists not gotten a mansion handed to them by a cured senior and his daughter/granddaughter in the first few chapters?
Here's the web novel raws: https://novel18.syosetu.com/n5291fa/
If you want to read about a protagonist making girls fall for his dick and cucking others, stay far far away from volume 3. I wish I had.