I'm with Papa Bear. Even if you get an offer letter tomorrow, tell company a that you need a few days to decide. Go to the interview with b, and if they bring it up, do let them know that you just received another offer. Two offers is your best negotiating tool. Even without 2 offers, you should make a long list of what you want (besides higher pay, there are things like signing bonuses, more vacation days, an account of discretionary money for travel and entertaining, professional development funds, etc.) and ASK about these things with any place that makes an offer. Don't expect to get everything you ask for, but only a fool just takes the first offer and asks for nothing. I've never heard of an offer being totally rescinded when someone was negotiating in good faith.