Tottenham confirmed on Thursday that a deal had been agreed with the England Under-21 international defender, with the move set to be sealed upon the completion of a medical.
Primeira Liga outfit Sporting have subsequently revealed that Dier was offered the chance to stay, but the club were forced to accept a €5 million offer from Tottenham as it met a release clause put in place by previous owners.
"Sporting Clube de Portugal announces that during [July] the club started negotiations with Eric Dier over the renewal of his contract," read a statement on Sporting's official website.
"Unexpectedly, Sporting, SAD was confronted by the player's father and official representative with a bid from Tottenham Hotspur, with that bid activating a clause agreed by previous boards of directors for the sale of the player for an amount of €5 million or more, unless the personal terms offered as part of the bid are matched.
"Due to the club's current salary policy, the respective balance within the squad and the on-going financial restructuring underway at the club, it was not possible to match the proposed terms.
"Furthermore, Sporting, SAD was also informed that the player did not want to remain with the club even on equal terms and that any equal or improved personal terms would not have been accepted.
"In light of the above, Sporting, SAD announces the complete sale of the economic rights of the player Eric Dier to Tottenham Hotspur for €5 million, without any future rights relative to the player.
"Despite the above mentioned events, Sporting, SAD wishes Eric Dier all the best in his career."
Dier moved to Sporting's youth academy at the age of 10 and played 13 league games last season after making his first-team debut in August 2012.
He will become Tottenham's third signing under new head coach Mauricio Pochettino following the arrivals of Ben Davies and Michel Vorm from Swansea City.
