Sporting Lisbon have lost their case before the Court of Arbitration for Sport against Doyen Sports group in the controversial transfer of Marcos Rojo to Manchester United.
The Portuguese club confirmed the judgement in a statement to the nation's stock market, saying they will appeal the verdict.
Sporting Lisbon president Bruno de Carvalho was challenging third-party ownership and the alleged share of the transfer fee Doyen Sports was claiming, which Sporting had refused to pay.
The CAS ruled that the club must pay the agency €12 million as 75 percent of the court's valuation of the deal.
Doyen Sports were alleged to have granted Sporting a loan of €4 million in exchange for 75 percent of Rojo's economic rights.
The Sporting president said last year that the club had retained the full transfer fee from Manchester United, with none of the fee going to Doyen Sports, adding that Sporting had returned the €4 million original loan to the fund, due to their belief that the contract was invalid.