Monaco vice-president Vadim Vasilyev says the Ligue 1 club turned down a succession of offers from Manchester United before agreeing to part with Anthony Martial for a fee of £57.6 million.
Martial, 19, signed for United earlier this week in a deal reported to be worth £36m up front.
French media reported that the
transfer fee could eventually rise to €80m (£58m) when bonuses were included, which would make him the most expensive French player in history.
And Vasilyev appeared to confirm that information as he said the final price would be £57.6m when "very realistic" bonuses are taken into account.
"Initially Martial was not for sale, it was absolutely not planned,"
The Sunquoted Vasilyev as telling Russian paper Sport Express.
"Since last week United started to make offers which we declined several times -- £21.6m, £28.8m, £36m, even £50.4m with bonuses.
"On Sunday the player got such an offer that he couldn't have declined. He asked us to find a solution and on Monday morning Manchester United made an offer to Monaco which the club couldn't refuse.
"It's absolutely unique. It's the price of Luis Suarez or Neymar -- the best players in the world.
"Right now English football is on a different financial level and if Manchester United decides to buy somebody, it's very hard to stop them."
On Thursday French newspaper
Le Parisien indicated the transfer included three bonus payments, each worth €10 million.
The first will be paid when Martial scores 25 goals in all competitions for United and the second will be triggered if he earns 25 caps for France between now and June 2019.
The third will come into effect should Martial win the Ballon d'Or as the world's best player between now and the end of his four-year contract at Old Trafford.
The paper also claims that Martial will receive a bonus of €1.15m every time that Manchester United qualify for the Champions League.