Spain's tax authorities are investigating the bonus Neymar earned when he extended his contract with Barcelona as well as his world-record transfer to Paris Saint-Germain, daily newspaper El Mundo reported on Monday.
The authorities are looking into whether the Brazilian paid tax in Spain related to the two deals, the newspaper said.
When contacted, the tax office refused to comment on the report.
Neymar was a tax resident in Spain in 2017, the year he transferred from Barcelona to Paris Saint-Germain on a five-year deal for a world record 222 million euros ($249 million).
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The 27-year-old Brazilian international and Barcelona are already battling in court over a bonus the player was paid when in 2016 he signed a new five-year deal with the Catalan giants just nine months before joining PSG.
A hearing over that case is scheduled for March 21 at a Barcelona labour court.
The tax authorities have asked the labour court for all the information it has regarding this case, the report said. The court declined to comment.
After Neymar agreed to move to the French side, Barcelona refused to pay the player 26 million euros which were part of a bonus it promised him when he extended his contract with them.
Barcelona are suing Neymar for 8.5 million euros for breach of contract and to demand he return the amount of the bonus money which it did pay him.
Neymar, in turn, is suing Barcelona to demand his former club pay his entire renewal bonus.
His is just one of several famous footballers who have fallen foul of Spain's tax authorities in recent years.
A Spanish court in January handed Portuguese striker Cristiano Ronaldo a suspended two-year prison sentence for committing tax fraud when he was at Real Madrid.
The player, who joined Italian side Juventus last year, also agreed to pay 18.8 million euros in fines and back taxes to settle the case, according to judicial sources.
Barcelona's Lionel Messi, once Ronaldo's big La Liga rival, paid a two-million-euro fine in 2016 in his own tax wrangle and received a 21-month jail term.
The prison sentence was later reduced to a further fine of 252,000 euros, equivalent to 400 euros per day of the original term.
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