I really think you have a fallacy of logic here. You don't have to pay income tax on it because you already have. You are esentially comparing your $50,000 income (having already paid tax on it) to someone making $50,000 before taxes and saying that your money is worth more. This is not a valid comparison. A valid comparison would be comparing your $50,000 which you have already paid tax on, to someone who made $50,000 after taxes. And the value of those monies are the same.
Where there IS an inequity is where your neighbor has his savings in a tax-deferred account, and will benefit from escaping taxation on it's balance when the FairTax is implemented. HE will have more value for his income than you will, because yours will have been taxed and his will not have been. This is an inequity in the existing system, as you have already paid taxes on your savings, and your neighbor has postponed it, to the point where he doesn't even have to now. The FairTax has no bearing on this inequity. If you both went and withdrew your balance and ran to Mexico never to return, the same inequity would exist.