Spurs announced on Thursday evening that they had reached an agreement with the Portuguese side, confirming that Dier would complete his move subject to a medical examination.
Sporting have since released a statement on their official website, and have slammed the actions of Dier's father and agent during contract negotiations.
"Sporting Clube de Portugal announces that during [July] the club started negotiations with Eric Dier over the renewal of his contract," read the statement.
"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 €5million 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 €5milion, without any future rights relative to the player.
"Despite the above mentioned events, Sporting, SAD wishes Eric Dier all the best in his career."