The phrase "one off deduction" is correct and usable in written English. You can use it to refer to a single, isolated deduction from a total, for example "the accountant made a one-off deduction of $50 from the total amount".
Switzerland has agreed to make a one-off deduction from all existing accounts held by people who are liable for British taxes but have not paid them.
The closest thing to a quantification comes in the Grattan Institute's submission to a Senate inquiry on tax deductions, which finds that limiting negative gearing could have a one-off impact on house prices of between 1% and 2%.
This is after a 1% annual deduction fee which, together with a one-off charge of £94.25 for each loan, is how Zopa makes its money.
The Treasury said that the rates of the withholding tax had been set slightly lower than the normal British ones to account for the fact that deductions will take effect sooner than would happen under the British tax system and that the one-off tax rate accounted for the fact that the taxpayer wouldn't have to fund lengthy investigations to recoup the money.
He was a one-off".
"These are one-offs.
One-off success?
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com