As deduction is a subtraction from income, before the tax rate is applied. It is not a tax credit or a refund. And your total itemizeds must be more than the Standard deduction. So a $100 donation is worth $25 in tax savings if you are in the 25% tax tax bracket. However, it can never lower your taxes to less than zero.
Conversely, itemized deductions phase out at high income levels.
