When I first got my dog, Toby, he came home with some parasites. The breeder doesn't usually extend the health guarantee to parasites because they could have been contracted after the animal left her care. I talked to my vet, though, and she said that with the type of parasite and stage of advancement, she could tell he had them for at least a week before I got him. I had my vet call the breeder. After the conversation, the breeder paid the vet bills.
It was only about $100 in this case, so she was less inclined to fight it, but I still think the first step is seeing if the vet can confirm how long the polyp has been around from the stage of advancement.
In any case, I'll be thinking about your kitty and hoping the surgery goes well.