squeak:
First of all, thank you for your clear and intelligent comments. We may disagree, but it’s nice to know we can do so intelligently.
It is an inarguable fact that you owe taxes to the government: however, the money is still not yet theirs. And as I said before, halachicly, the only reason one is michuyav to pay up a debt owed to a goy is because it will be a chillul Hashem if you don’t. (which just goes to show that not making a chillul Hashem is a halacha, not just a nice thing to do) If the goy would forget that you owe him the money, you are not mechuyav to pay him back since no chillul Hashem will take place. This is the case here, since Uncle Sam has no idea you ever owed him anymore than you claim you do. Again, it is still assur because of sheker, but it’s just not geneiva. And btw, I didn’t make this up; I heard it in a halacha li’maseh siur from a prominent poissik.
As for the argument that not paying taxes causing others to pay more, I simply don’t think that is factually true. The government does not charge taxes based on how much money it does or does not have: if it did, we wouldn’t be stuck with a 13+ trillion dollar national debt now, would we?
Oh, and your analogy about the old lady and the lawyer is very fitting, perhaps more than you realize. You see, in a case where a goy hands you back extra change, you are not michuyav al pi halacha to return it. It may be a kiddush Hashem and a nice thing to do, but it’s not a chiyuv.