The Electoral Commission fined the Constitutional Research Council (CRC) for failing to notify it about the donation.
However the commission later found that the source of the money was permissible.
The DUP spent most of the money on pro-Brexit advertising.
The information was released in a letter by Electoral Commission and released by the Good Law Project, which has threatened to take a judicial review over the matter and is trying to halt Brexit.
The commission said the CRC “had no reasonable excuse for these failings” the BBC reported.
The CRC is chaired by Richard Cook, a former vice chairman of the Scottish Conservative and Unionist Party.
The BBC contacted him but he declined to comment.
Of the £435,000 donated to the DUP, £425,000 was spent on the Brexit campaign, covering advertising in the Metro newspaper in support of Brexit, promotional materials and on a consultancy firm.
The DUP says it complied with electoral law at all times and that it had nothing further to add to previous statements on the matter.
Under electoral law, the CRC should have notified the commission of the donations. An investigation by the commission found that the CRC “had no reasonable excuse for these failings”.
In the letter to the Good Law Project, the commission said: “The CRC told us it was not aware of its reporting obligations but that it now recognised that it should have notified the Commission and the error was inadvertent.”
The commission fined the CRC £6,000, but said it also verified that it and its donors were permissible.