I have had a similar experience. I have three Burmese, one of whom wanders a lot. They are microchipped and neutered. A few months back I got a call from a local vet, saying that someone had bought in my cat as he was hurt - been in a fight.
Prior to scanning, the vet said that they had to register him before they could treat him, or he would be declared a stray and kept by them!
They registered him, THEN, the vet scanned him, and called us.They wouldn't tell me what was wrong with him, but wanted me to authorise payment on my card to treat him. I said is it life threatening?
I refused treatment on the basis that this particular vets charge like the light brigade, as I previously had them all neutered there, lots of extra unnecessary charges for blood tests etc. My vet visits me because I had 5 Burmese and it was easier than catching them all for jabs, and we do it on the dining room table!
The vet was not happy that I didn't want him treated by them, and said we have treated him before you know. I said yes, and I no longer bring him to you!
The people bought him back to us, I thanked them, and said how lucky you have a cat carrier, do you have a cat? They said no, they had been and bought it to take him to the vet. They live 5 doors away, but we are on a country lane so that is a distance, but we do have a large enamel painted plaque on the gate post with three Burmese cats on it! How they got him in it beggars belief!
Did I pay them for the cat carrier, which was not my choice, of which I already have 5? Yes I did, and bought a bunch of flowers and delivered them. I would have been a bit pissed off if they had paid huge sums of money, as is likely at this vet, and would probably felt obliged to pay them back, as they were well intentioned.
I treated the cat's cut head myself, I am used to doing it, but they run a mile when they sniff the TCP!
He still visits them, but at least they know he is ours now. I do not use collars as they have got hooked up in trees, come back with a leg hooked through etc, and these are quick release collars too.