Sporting CP rejected Tottenham's best offer for Bruno Fernandes because of doubts over the English outfit's improbable trophy plans, the Portuguese club's president said.
Frederico Varandas revealed Spurs offered a deal worth up to £58.4million (€65m) for midfielder Fernandes, who was also linked with a move to Real Madrid on deadline day.
The Portugal international last month said Tottenham had "made a lot of proposals" to prise him from Sporting, where he has played since 2017.
According to Varandas, one involved £18million (€20m) in add-ons, to be paid in the event Mauricio Pochettino's men won both the Premier League and the Champions League.
"It was 'impossible' to retain Bruno Fernandes, but we did it," Varandas told Sporting TV.
"We prepared for his sale by putting a fair price on his worth and we had bids.
"Tottenham [offered] €45m plus €20m in objectives - winning the English league and the Champions League.
"Finding these objectives difficult, I chose not to accept."
Tottenham reached the Champions League final last season but lost 2-0 to Liverpool. They last won the English league title in the 1960-61 season.
Negotiating a deal for Fernandes in January is unlikely to prove any easier, with Sporting preparing to offer their 24-year-old captain a new contract.
"Bruno Fernandes proved to be not only a great player but a great professional and captain," Varandas said.
"The day I turned down the Tottenham proposal, I came to the club and talked to him, told him why I declined and why he would stay here until the end of the transfer window and he understood perfectly.
"In addition, he also realised that the value they gave to Sporting CP was not fair.
"His contract will be reviewed, not because it was planned, but because he deserves it."