The tax deduction is deducted from your taxable income, is what he said. His math checks out.

Explained differently: A union membership costing 700 only costs you 490, assuming your tax is 30% and that it is deductible where you live.

Meanwhile, the game console still costs 700.

You are allowed to deduct the money you spent on union dues from your income, thereby lowering your taxable income. So on that portion of income that you deducted, you don't pay taxes. That's how all deductions work.