Cat was there first. I would re-home the dog (responsibly, of course). Perhaps he could go back to the organization he came from? It's not worth endangering your cat's life. And Craig's List is full of cats and scary people who want free animals for the wrong reasons.
I had a cat aggressive foster dog for 2 months. I divided my house in half. The dog stayed upstairs crated and behind a closed door and the cat stayed downstairs. I also put a gate at the top of the stairs. I divided my time between upstairs and downstairs so that they both got equal time. I would not have lived that way forever though because it was exhausting and stressful.
Ruth & the 4 Legged Rescue Gang
Rafi the malaroo
Gio & Varda, the krazy kittens
....In my heart: