legal(?) obligations of a BARC early adoption

my partner and i recently brought home a kitten from BARC (early adopt) under a (signed) agreement to bring him back for his vaccines and neutering at the appropriate age. they sent us home with a “3 week old” kitten and a liability warning that “kittens sometimes just die”. while not inherently a red flag at the time because this is true, in hindsight it feels weird. two days later he has bloody diarrhea and is lethargic. we took him to the vet and 400 dollars later he has giarida AND feline parvo. both of which have high mortality rates in kittens. the vet also said he is probably 5-6wks old. he has since been on medication and is doing far better and it appears is on the upswing (seemingly due to our early intervention)

we are currently at a loss for what to do. we don’t exactly feel comfortable reintroducing him (and by proxy our other cat) to the environment at BARC where he either caught or was not tested for these diseases. (we do fully intend to have him vaccinated and fixed, as our other cat is). what legal grounds would BARC have to reclaim this kitten (or how would they do so) if we were to refuse to bring him back? would they accept proof of vaccination and neutering from a different clinic?

TLDR; cat from BARC got sick without adequate warning. don’t want to bring him back there to get fixed. what problems might this cause us?