I was going to ask about payment plans - that sounds good.
The other thing was whether the vet had spoken to you about weighing up the risks of surgery vs the chance of natural healing. I have a cat that broke his leg on one of the joints and the vet gave us three options - amputation, surgery or let it heal naturally. They advised the surgery was a difficult one so may end in amputation anyway, so we went with natural healing. It was about 10 years ago now so I can’t remember how long he rested but it doesn’t seem like it was that long. He has had full mobility since. It was an easier choice for us because the chance of amputation was high and that seemed worse than a wonky leg.
Good on you for looking after him and damn those sellers for letting it go and then trying to pass it off as nothing to start with. At least they did give your money back but still, it doesn’t really cover it. I hope the surgery goes well.