Barcelona have been fined €602 ($669) by the Royal Spanish Football Federation (RFEF) for approaching Julián Álvarez before notifying Atlético Madrid of their interest.
Barça plan to appeal the decision, a source said.
The Spanish champions made several attempts to sign Álvarez in the summer as they sought a new striker to replace the outgoing Robert Lewandowski and Ferran Torres.
Their pursuit of the Argentina forward angered Atlético, who accused Barça of making an illegal approach for the player and subsequently reported them to both the RFEF and FIFA.
FIFA declared it had no jurisdiction over the case, but the RFEF announced a small fine this week after finding Barça made contact with Álvarez's agent at the time, Fernando Hidalgo, before Atlético.
The RFEF's investigation established that a conversation took place between Hidalgo and Barça sporting director Deco on May 15, with the Catalan club's first written communication with Atlético following on May 28.
However, the infringement was only considered a "minor regulatory breach," with it falling under Article 133 -- instead of Article 93 -- of the RFEF's Disciplinary Code and, therefore, carrying a smaller punishment.
Article 93 is applied for "conscious and repeated breaches" of the RFEF's regulations.
Barça offered €100 million ($111m) for Álvarez and left the offer on the table until the end of the transfer window, but Atlético CEO Miguel Ángel Gil Marín repeatedly insisted they would not sell to their LaLiga rivals.
Atlético also turned down a €150m ($166m) offer from Real Madrid for Álvarez during the summer, with Arsenal and Paris Saint-Germain also expressing an interest in signing the forward.
Álvarez had revealed a desire to leave, telling ESPN during the World Cup that he felt a transfer would be best for all parties. Sources told ESPN he wanted to join Barça.
His push for a transfer saw him booed by Atlético fans at the start of the season, although bridges have started to be mended in recent weeks.
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The 26-year-old has scored in each of Atlético's last two games, against Stuttgart and Deportivo A Coruña, and after netting the winner in a 1-0 win against the latter on Saturday, he apologised to supporters.
"I wanted to say sorry to our fans for my comments during the World Cup," he told Movistar of his decision to speak out in search of a move.
"It was a mistake -- the way I said what I said and the timing. I understand why the supporters are angry. If I were a fan, I'd be angry too. Especially given how they've supported me since I arrived."
Atlético have won eight of their opening 10 LaLiga games and are second in the table, just two points behind leaders Barcelona.
The two teams will meet for the first time since Atlético reported Barça for their Álvarez pursuit at the Metropolitano in Madrid on Nov. 8.