Sinn Fein will not have to pay any inheritance tax on a £1.5m gift from an English mechanic who bequeathed the money in his will.
The donation, revealed this week, is believed to the biggest ever made to a political party in Northern Ireland.
Under HM Revenue & Customs rules, any political party that has at least two MPs in the House of Commons is not liable to pay inheritance tax on such a gift.
Sinn Fein has seven MPs but because of a Westminster abstentionist policy they do not take their seats in the house.
A political party also qualifies for exemption if it has one MP and received no less than 150,000 votes at the last general election.
Without exemptions, anyone who is left £1.5m would be required to pay around £470,000 in inheritance tax.
The party's windfall was made public on Thursday by the Electoral Commission.
The benefactor has been identified by a source close to Sinn Fein as former mechanic William E Hampton.
Mr Hampton was born in London and died on January 11, 2018 at his home in Pembrokeshire, Wales, at the age of 82.
The pensioner, who wasn't married and had no children, also left money to friends and acquaintances.
However the main beneficiary was Sinn Fein.
It is understood that he spent some time living in Ireland and was a long-term supporter of the party.
The money was paid to Sinn Fein in instalments of £1m and £500,000 in April and May this year.
"He made the will himself, and had it drawn up a few years ago. He's obviously been a supporter a long time and planned ahead," the source said.
"The party have known about him for a long time and it wasn't exactly new information to the party as the will had been made a few years before.
"The death, however, came out of the blue and was unexpected. The death certificate said he died of respiratory failure, and he was quite frail and elderly, so it was probably old age."
Sinn Fein said: "We're obviously pleased that he has chosen to bequest this sum to the party and it's a positive boost to Sinn Fein in working towards Irish unity and towards our political objectives.
"We are in full compliance with the requirements of the Electoral Commission on all of this."
The DUP previously held the record for the largest donation given to a political party in Northern Ireland after it was given £435,000 from the pro-Brexit Constitutional Research Council in 2016.