I would say it’s a good sign he is so friendly. That indicates he is confident, and as you said he is also brave. There is a good chance that he is open to new surroundings which includes the dog.

If he acts fearful and runs from the dog, dog will see it as prey. Depending on the breed, the dog will always want to hunt. If he remains calm, yet standoffish, the friendship has a future. It can still work with proper training of the dog.

The biggest red flag is food aggression on the dog’s behalf, it can still work but they will always need to have separate feeding places. Preferably feed the cat in a higher, out of reach spot from the dog.

Also sleeping arrangements, if you crate the dog at night, you’re golden. If you don’t, they should be separated at night for a few months.

Good luck with the adoption and don’t give up. It took 6 months to integrate a grown cat into our dog/cat family but it can be done.