I don't think the puss in question would be abandoned as such - he would stay in the home he's used to, with what sounds like an owner who would spoil him rotten.
A very difficult decision. I think I'd take the money, give a puss from the shelter a home, and donate a huge wad to the sanctuary. That way, lots of cats would benefit from this very unusual scenario.
In theory that makes sense to me. However, in the case of my own puss (RIP Puss, so sadly missed) I don't know if I would've been able to do it.