Carl Frampton's wife Christine and his trainer Shane McGuigan had a row in Las Vegas, which was followed by a threat of legal action over unpaid money, the High Court has heard.
On day 19 of the multi-million pound legal battle between Mr Frampton and his former manager Barry McGuigan, promoter Blain McGuigan said his younger brother and Mrs Frampton had exchanged words after the Belfast boxer lost to Mexican Leo Santa Cruz in a WBA super-bantamweight world title rematch in January 2017.
Mr Frampton's barrister, Gavin Millar QC, told the court his client only got paid for the first Santa Cruz fight (which he won in New York in July 2016) after emails were sent by his accountant advising the McGuigans that legal action would ensue.
Mr Frampton (33), a dual-weight world boxing champion, is suing Barry McGuigan and Cyclone Promotions for £6m alleged withheld earnings.
In a counter-suit, Barry McGuigan (59) is counter claiming against his one-time protege for breach of contract when he ended their partnership in 2017.
Both men deny the respective allegations against them.
Blain McGuigan, Barry McGuigan's eldest son, told the court: "After the Santa Cruz fight on January 28 I knew that there had been argument between my brother and Carl's wife and then the next time I heard of any dealings between Carl and his accountant and us... there was a threatening letter."
Mr Millar asked: "A threatening letter or email?" to which Blain McGuigan replied: "Email."
The barrister then read out an email claiming that Mr Frampton was annoyed because he still hadn't been paid for his fight with Leo Santa Cruz in July 2016.
It also said that the Tigers Bay native had been advised to allow one week for this payment to be made before taking legal advice.
A second email was read to the court that said "should this payment not be forthcoming in the next seven days he should take immediate legal advice".
Mr McGuigan replied that after "these emails and following probably a heated phone call, which is probably what he was referring to, he was paid his money. That's what I know".
Mr Millar said: "He was paid his money after those two emails were written saying it was going to be directed towards his lawyers to get his payment for the Santa Cruz 1 fight."
As cross-examination drew to a close, Mr Millar said that alleged breaches of duty entitled his client to terminate the International Promotional Agreement (IPA) with Cyclone to which Mr McGuigan replied: "I disagree."
Mr Millar said: "You betrayed his trust in you". Mr McGuigan replied: "No, I did not".
Mr Millar said: "You failed to conduct the financial arrangements surrounding the rights you say you promoted for him in a transparent and open manner".
But Mr McGuigan insisted: "Carl was always paid exactly what we discussed and he always knew the workings of each deal".
The barrister then put it to Blain McGuigan that he had concealed facts from Mr Frampton regarding the true income from the fights, to which Mr McGuigan replied: "Nope. Everything was discussed with Carl. He was completely informed and he never made any complaint throughout his whole time with us".
Mr Millar said: "You concealed the true profits and misrepresented his fees by inflating expenses and the cost of fights".
Mr McGuigan responded: "That's not true".
Mr Millar then asserted: "And you deliberately delayed large payments to him for lengthy periods so that you could retain the benefit of these monies".
Mr McGuigan replied: "I've explained the whole process. Carl didn't make any complaints at the time".
Mr Millar said: "Those were serious breaches of the trust he imposed in you as the son of his manager in these promotional arrangements".
Mr McGuigan replied: "I disagree. He knew me as a promoter. We were friends throughout and we'd a very good relationship and discussed pretty much every detail of every show". Earlier Mr Justice Huddleston heard that Mr Frampton's promoters had $850,000 (£648,000) that they hadn't told him about when the relationship between boxer and manager broke down.
Mr Millar put it to Blain McGuigan that the family's promotions firm had held on to the money "hoping things would be left" following the parting of the ways in 2017.
Blain McGuigan replied that such an allegation was "not true".
Mr Millar examined a document detailing the finances from the boxer's second fight with Leo Santa Cruz in January 2017.
Mr Millar said: "At the point the relationship broke down, you had the $850,000, you hadn't told Mr Frampton about that."
Mr McGuigan replied: "We told him that we received our part of the payment. He was aware of that. He'd already received his purse."
Mr Millar countered: "He'd received that sum of money representing the net value of the $1m purse, and that was where you were hoping things would be left. He'd leave the arrangement with that, and you'd keep the $850,000."
Mr McGuigan said: "That's not true. We were working on the overall income after the event."
Mr Millar replied: "But you wanted to make sure that he didn't know about the $850,000, because that would become an issue as indeed it is in these proceedings."
But Mr McGuigan, who was on the stand for a fifth successive day, insisted: "No we were completely clear with Carl about how we were still settling the event."
The hearing continues.