you have a charitable donation of 10M so you can deduct 10M in tax, at a cost of 4M. Tax rate is not relevant, you are subtracting 10M of your tax liabilities
In which country is this? For example, https://en.wikipedia.org/wiki/Charitable_contribution_deduct... says "The taxable income of the donor is reduced". To be specific, this would mean that instead of paying the tax rate on 100 million, you now pay it on 90 million.