The phrase "a one off sum" is correct and usable in written English.
It can be used to refer to a single payment or amount of money that is not expected to be repeated.
Example: "The company offered a one off sum to employees as a bonus for their hard work during the project."
Alternatives: "a one-time payment" or "a single payment".
That £1.1bn is a one-off sum that comes from the fines imposed on banks after the Libor rate-rigging scandal.
At present this is a one-off sum of about £2,300 for a four-year degree, paid once graduates are earning more than £15,000.
In real life, Admiral Lindsay left a one-off sum of £1,000 to each of his other children, and not a bean to Dido.
The supermarket site, on which Sainsbury's has taken a lease, Spurs preferring the regular cash flow rather than sell for a one-off sum, is also owned by TH Property Ltd in the Bahamas.
It hopes to push forward the redevelopment of the Grade II-listed Municipal Offices "as soon as possible", and intends to offer it through a ground rent or performance share scheme rather than sell it for a one-off sum.
In the 1960s Germany paid the British government a one-off sum so that people who had suffered from Nazi persecution in German prisons and concentration camps could be compensated.
If you choose to defer your pension you will be entitled to a higher weekly amount later on, or a one-off lump sum payment.
The only products currently on the market are immediate needs annuities, which involve people paying a one-off lump sum - often about £100,000 - when they start needing care.
"They can keep it within a pension, and pay income tax at their highest marginal rate (ie, up to 45%) on the income they choose to withdraw, whether this is paid as a one-off lump sum or as a number of payments," says Hargreaves Lansdown.
Even a one-off lump sum could make a difference.
Accounts can be opened with a one-off lump sum of up to £1,000 in addition to the monthly maximum.
