Denis O'Brien snr told the tribunal he was "livid" in September 2002 when £2.5 million sterling was sought from him in what he considered a blackmail attempt.
Mr O'Brien was answering questions from John Coughlan SC, for the tribunal, about a mediation hearing Mr O'Brien attended in London arising out of the deal involving Doncaster Rovers Football Club Ltd.
He was giving evidence for the third day as part of the tribunal's inquiry into the Doncaster deal which Mr O'Brien's son, Denis O'Brien jnr, has said was exclusively his and had no connection with former government minister Michael Lowry.
After the £4.3 million sterling transaction in 1998 there was a dispute between the purchasers and the vendors over money that had been kept in a retention fund.
The matter was on the way to the High Court in London when both sides agreed to a mediation hearing.
A note of the meeting, read out yesterday, recorded that during the mediation hearing a side meeting in the absence of lawyers was requested by the vendors, Ken Richardson and Mark Weaver.
The note-taker did not attend the side meeting but an accountant working for the O'Brien family, John Ryall, produced an aide-memoire afterwards.
His note recorded Mr Richardson saying he would sell his company, and its files concerning the Doncaster deal, to Mr O'Brien for £2 million sterling plus the £440,000 sterling in the retention fund.
When the mediation resumed afterwards the note recorded Mr O'Brien as saying he would not "give in to blackmail" and that he had never met a more unstable person than Mr Richardson.
"DOB said he had nothing to hide and that he was not afraid. Now he wanted to go to war." When Mr Coughlan said the note recorded Mr O'Brien as being "clearly furious" after the side meeting, Mr O'Brien said: "I was livid."
The mediator, Michael Kallipetis QC, was recorded as saying Mr O'Brien had been very patient during the side meeting "and if he had been in DOB's shoes he would have 'thumped him'," meaning Mr Richardson.
The dispute was eventually resolved with the vendors receiving a sum in excess of £700,000 rather than the £1.2 million that had been sought.
Mr O'Brien snr subsequently reported what had happened at the side meeting to the police in London, complaining of attempted blackmail.
He said to Mr Coughlan: "If I was trying to hide anything, particularly in relation to Mr Lowry, would I have really taken the steps I did?"
The tribunal also outlined how it had been informed that the police in London had concerns about documents linked to the blackmail complaint being released to the tribunal.
The tribunal subsequently learned that this was not the case.
Mr O'Brien's evidence to the tribunal is expected to end today.
