Who's paying for whose mistake?

Professor Don Boudreaux makes a helpful observation about Representative Charlie Rangel's attempt to eliminate the Alternative Minimum Tax. Framing the mission creep of the AMT as a "mistake" that Congress appears ready to correct, he notes:

Given that the current operation of the AMT is a mistake, why do Rep. Charles Rangel and so many others talk of the need to "pay for" fixing the AMT? A merchant who mistakenly overcharges customers is obliged to refund the money and stop overcharging, period.

The problem, of course, is that Congress has already spent the money. But I guess they could just dip into those unsecured Social Security monies to pay for things like they did in the good ol' days.