The immigration reform proposal from the Gang of Eight contains a provision that could require immigrants who want to seek citizenship to pay off any tax liabilities accumulated while working illegally.
There are a variety of reasons this proposal is unrealistic. Most immigrants probably don't owe any back taxes, either because they've already paid them or because their income is too low to owe any taxes. What's more, figuring out a tax-dodging illegal immigrant's past income is a more or less impossible task. It's not like there are extensive records on off-the-books wages.
But let's ignore all this and pretend for a moment that we actually identify illegal immigrants with large tax liabilities. How are they going to pay these off?