The US tax write-off for donations doesn't give you a net profit; it just excludes the donation from your taxable income, as if it had never passed through your hands. And even that applies incompletely: write-offs are taken instead of, not in addition to, the standard deduction. So if the employees were going to deduct a bunch of stuff anyway, then they're indifferent between the two pay structures; and if not, then they'd be better off if they were just payed 4% less and the company did the donating.