If I'm understanding correctly the claim is that it was paid by the corporation which treated it as an expense that reduced corporate income for tax purposes. The problem I would think would be is that for a private company like The Trump Organization how can you separate Trump's income from the company's? And unless Trump deducted it then how do you prove fraud?
This might be where Cohen comes in. But I doubt Cohen did Trump's taxes. And, I doubt there was a line item deduction for "Pay-off Porn Star" in any of the tax filings. So, just assuming stuff, if Cohen "says" Trump filed it as a legal or business deduction, then it must be true. If this is the case, then what, Trump "told" Cohen how this was accounted for on his taxes? I doubt that.