Spurs reached an agreement - subject to a medical - over the transfer of the centre-back on Thursday, but his father has been criticised for forcing the Portuguese club's hand
Goal revealed on Thursday that Spurs had agreed a €5 million deal with Sporting for Dier, who was born in Cheltenham but has spent the last decade in Portugal.
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"Sporting Clube de Portugal announces that during [July] the club started negotiations with Eric Dier over the renewal of his contract," the statement read.
"Unexpectedly, Sporting were confronted by the father and the official representative of the player with a transfer offer from Tottenham Hotspur, who referred to the current administration an agreement signed by previous directors and administrators, which obliged a sale of the player for a sum not exceeding €5m, or for [Sporting] to equal the proposed salary, which is impossible due to the wage structure in place, the ongoing financial restructuring and necessary sustainability of the club.
"Additionally, Sporting were informed that the player does not want to stay at the club even if the proposed salary was matched. And even if Sporting received a superior offer, whether in terms of a transfer bid or salary, from another club the player would not accept it.
"Given the constraints outlined above, Sporting announces the sale of all economic rights for the player Eric Dier to Tottenham Hotspur, for the value of €5m net, without any future rights or guarantees.
"Sporting wishes Eric Dier all the best in his career."
Dier, 20, will become Tottenham's third signing of the summer, following left-back Ben Davies and goalkeeper Michel Vorm, both purchased from Swansea City.