You call A on Monday, say you are delighted to get the offer. Ask how long you have to decide. Ask for it in writing. If you have a spouse say that of course you need to talk this over as a couple and you are sure your spouse will have some questions about the town. Your goal is to make it sound like you are going to take the offer. Say nothing about your upcoming interview at B.
Go on the interview to B. Mention that you just got an offer from another school--make it sound like it was within the last 48 hours. Do your best but don't feel pressured--you already have a job.
In all likelihood you will have to give A a yes or no before you know if B is going to make an offer. This sucks but is typical. At least you got to check out B. Let us know how it goes.
As usual, larryc is right on the money.
I have just one small piece of advice to add. You're probably a reasonably decent human being. As such, you will have twinges of guilt should this get to the point at which you are forced to "accept" the offer from School A. Negotiate with School A like this is the job you will die doing, even if you already
know that you will accept an offer from School B if you are offered the position. Don't sign anything unless you absolutely have to, but don't sweat making a verbal committment to School A.
This sort of thing happens all the time, in other words. And even though you will feel like a heel doing it, and even though it will violate or call into question every good liberal sensibility you have, and even though it's an affront to your entire value system, try as hard as you can not to let that get to you. This is a tough business, and the fact of the matter is that any one of these schools would leave you unemployed without batting an eyelash should some Deanlet decide over his morning coffee that eliminating a tenure line at the last minute might give him a positive line of conversation at his weekly Friday morning golf game with the Provost.
Be tough. No one else is looking out for you.
