Mr Sean Fleming TD, the former Fianna Fail party treasurer, will give evidence in public to the Moriarty tribunal next week when the controversy over the missing £75,000 donation to Fianna Fail will be raised.
The Taoiseach, Mr Ahern, according to sources, discovered only this week that £75,000 of the £100,000 given by a property developer, Mr Mark Kavanagh, to Mr Charles Haughey at the 1989 general election was not passed to Fianna Fail.
Mr Ahern, they said, was told some years later, probably in 1996, that Mr Kavanagh had complained of not receiving an acknowledgment of the money but was never told the amount involved, except that it was "substantial".
Mr Ahern has not been called by tribunal lawyers to discuss the money.
The tribunal starts public hearings on Tuesday. Earlier this week Mr Fleming appeared before it in private session when he said that a donation of £25,000 had been received from Mr Kavanagh at that time.
He was told by tribunal lawyers that they were aware of four separate payments of £25,000, a cheque and three bank drafts for similar amounts.
Mr Fleming told The Irish Times yesterday that while the £25,000 donation had been listed in Fianna Fail files as anonymous there was a photocopy of the cheque on file elsewhere and it was known who had given it.
"The word anonymous was used at the direction of the Taoiseach [Mr Haughey] but that is not really a proper description of how it was. We had the photocopy of the cheque and the money had been lodged and recorded," said Mr Fleming, adding that at the time it would have been considered a "considerable" donation.
However, he did not wish to say how it compared with other donations at the time by revealing higher figures that had been given.
Party sources have said that in 1996 when a Fianna Fail fundraiser, Mr Eoin Ryan snr, sought a donation from Mr Kavanagh, managing director of Hardwicke property development group, Mr Kavanagh complained that he had received no receipt for his 1989 donation.
They said there was no mention of Mr Haughey and no amount was disclosed. Mr Fleming said that from what he had heard of Mr Ryan he was not surprised by this.
Mr Ryan was not available for comment yesterday, but it is understood he has not been contacted by the Moriarty tribunal concerning this issue.
Mr Kavanagh, whose company was part of the consortium which built the Irish Financial Services Centre, was not available for comment yesterday.