The case has put the European Union's powerful executive on collision course with the 11-times European champions.
Real Madrid said Tuesday it would appeal against an EU decision requiring the club to repay 18.4 million euros ($20 million) in alleged illegal aid from a land deal.
"We are going to file an appeal, and I am absolutely convinced we will win," Real president Florentino Perez told a press conference.
The case has put the European Union's powerful executive on collision course with the 11-times European champions and one of the world's wealthiest clubs.
The European Commission accuses Real and Madrid city council -- which at the time was run by the conservative Popular Party -- of illegal payment related to the sale of a plot of municipal land.
In 1998, the club paid the city 595,000 euros for the land but the deal ultimately fell through because of technical problems. In 2011, the city paid Real 22.7 million euros in compensation.
According to an assessor hired by the Commission, this figure was over-stated by a massive 18.4 million euros and thus amounted to illicit aid.
Perez, who was speaking at a presentation of Real's plans to modernise the Bernabeu stadium, dismissed the far lower valuation of the property.
"It's meaningless when you see it's been carried out by a company from Barcelona that furthermore is tiny and not an expert," he argued.
"There's nothing" in this case, Perez said. "We are going to file an appeal against the European Commission's assessment."
Since 2015, city hall was been run by the left. On August 29, it said it would pursue Madrid for payment of the 18.4 million.