If you pay Hillary 2.5 million in cash and then have to write it off, you have a loss that offsets 2.5 million in profit. If tax rates are 30%, you avoid $750,000 in taxes. So are still out $1.75 million after the write-off.
If you are going to have an expense, it's better to be a write off because it only costs you 70% on the dollar after the tax savings rather than 100%. But it's always better not to have the expense in the first place and have to pay the taxes.
The money paid to Hillary was not an expense, it was an investment & a payoff rolled into one. A payoff for favors done by X-42 and an investmest for future favors from (s)Hill.
I suspect, by the time this has played out, the executives who decided to front this money for Hillary will have gotten their investment back tenfold or more.