Former Fianna Fáil fundraiser Des Richardson has denied using a bogus invoice from NCB Stockbrokers to cover a £5,000 payment to Bertie Ahern.
But Mr Richardson says he cannot say how the donation was made after a request to former managing director of NCB Stockbrokers Padraic O'Connor.
Mr Richardson has told the tribunal that Mr O'Connor of NCB agreed to give a £5,000 contribution towards a legal bill facing Mr Ahern.
But Mr O'Connor says he was told it was for Mr Ahern's constituency expenses.
The tribunal heard that NCB paid £5,000 plus VAT to Euroworkforce, a company formerly owned by Mr Richardson, for a health and safety survey that was never carried out.
Mr Richardson said he had not seen that invoice, but cannot pin down now how the payment was made.
He said he first became aware in July 2006 that Mr O'Connor did not agree with his explanation for the donation.
Mr Richardson said he first informed Mr Ahern of this in September or October last year around the time details of his goodwill loan appeared in the newspapers.
Dollar donation not lodged in account
Earlier, Mr Richardson told the Mahon Tribunal that a $10,000 donation he received in Manchester was not lodged to the party account.
Mr Richardson said he used the money given by an British developer to pay debts incurred in Fianna Fáil fundraising.
Mr Richardson said businessman Norman Turner gave him the money in an envelope during lunch in a Manchester hotel before the two men attended a football match at Old Trafford.
Mr Richardson said Mr Turner, who the tribunal heard was planning a big development in Ireland, wanted the payment to remain confidential and not recorded.
Mr Richardson said he did not realise the money was in dollars until he got back to Dublin.
He said the money was not lodged to the usual Fianna Fáil account held in the name of Albert Reynolds and Mr Ahern at Bank of Ireland Baggot Street.
It was converted into £6,780 and use to pay fundraising expenses to Castlemanor Softwear who supplied gifts and prizes for party outings and the Galway races.
He said he informed Fianna Fáil of the donation in a hand written note.
Mr Richardson said he had planned at one stage to put the money towards a £12,500 personal debt incurred in a fundraising scheme involving a company called Trent Valley but in the end paid this debt out of his own money.
