You don't put it anywhere. There is no "write-off." You report only the income that you actually received, not what you asked for but did not receive. That way you are only paying tax on the income you actually receive. The amount that you did not receive is already subtracted from the income you report, so you are not taxed on income that you didn't receive. Since it's already subtracted from the income you report, you can't deduct it again.
