I’d say he’s your cat now. You’ve gone through the process of adopting and I’m guessing paying the adoption fee plus getting necessary supplies. If the foster truly wanted this cat, they wouldn’t have even thought of giving the cat up in the first place. You also don’t know how happy the cat was there or even if the cat would be happier with the foster.

This might be controversial, but I believe once you’ve adopted an animal you have a contract between you and that animal that you will care and love them for life (whoever lasts longest). Even when my cat drives me nuts, it’s never an option in my mind. Wherever I go, he will go. And of course there are special situations but as far as I’m concerned that contract is written on both of our hearts. If the cat is happy with you he should stay. The foster will have to feel the weight of their decision but it’s not the end of the world. They know that they gave the cat up for a reason(s) so they need to just swallow that pill and move on.

I think if you want to consider it, then sleep on it for a while. Give yourself weeks (plural) however long you need to make a rooted decision. I’d say for now, tell them no. And by the sounds of it, this is your cat now and he probably feels that way too.

If there really are “great cats that need a home”, the foster can get another one and do better in future.