When you say it's pretty serious, did the vet basically tell you your rabbit was not going to make it through? I find it very...interesting that the pet store would ask for the rabbit back and just give you another one like it's no big deal. I don't want to jump to any conclusions but it sounds like your rabbit was let go. I'm not sure how Ireland is, but here where I'm at, there are some pet stores whom are notorious for not providing adequate care for animals in general and you unknowingly purchase a pet and end up with a large vet bill. That happened to my friend who adopted her puppy from a "legit" pet store. A week later, her pup had to be rushed to the ER because she had Parvo. The pet store never mentioned anything. Took $1,500+ from her and she landed with an even bigger vet bill. If you're able to, see if you can reach out to a rescue or a shelter instead where there are so many rabbits who deserves a good home as well.