Your question is like a breath of spring air after a long winter of lowfat this and reduced-calorie that. You are totally out of synch with the rest of the country and we love you for it. We would probably just substitute a half cup of butter for the oil and see how that worked. If the brownies turned out too gooey, we would move on to Plan B.
But you are right, butter is about 80% fat and remainder is water, milk solids, and, in the case of salted butter, salt. You could go to the trouble of clarifying the butter, but why not just use 10 tablespoons of butter and only 2 tablespoons of water instead? That's Plan B, by the way. (Plan C is to clarify the butter, in which case you'll need to clarify about 10 tablespoons in order to wind up with half a cup. We hope you don't have to resort to Plan C.)