How?
Without admitting their dual identity, any attempt by Bruce Wayne to pay Batman's taxes, indirectly, is tax fraud.
Batman had honor and he was not going to cheat and not pay his fair share of taxes. What he likely did was look at how much money was spent on buying all the stuff that he needed to make his Bat stuff. Lets say it all came out to 20 million dollars. He would then go buy something else for 20 million dollars just so the government could tax it and gets it's fair cut of 20 million dollars.