That $540 is the net result of reducing one's taxable income by $2000.
Remember that a deduction does not mean "deduct what is owed to the Gov't," (that's a tax credit) but "deduct the amount of taxable income that is used to calculate how much is owed the Gov't."
Let's pretend this is the guy's only deduction (yeah, I know, he'd do better with a standard deduction, but whatever). His AGI (adjusted gross income, which is used to calculate what your tax rate is) is $50,000. That puts his tax rate at 27%. So that means his starting tax bill (how much taxes he's supposed to pay the gov't) is $13,500.
From that $50,000 he can deduct certain expenses, such as charitable contributions. By contributing $2000, his taxable income gets reduced to $48,000. Multiply that $48,000 by 27% and you have a tax bill of $12,960.
$13,500 - $12,960 = $540 off your taxes for donating $2000.